EMBRYOLOGY is the science which treats of the develop ment of an individual animal from its beginning as an egg to a period arbitrarily determined when it has attained or nearly reached its adult structure. With perhaps a very few exceptions all multicellular animals, both Parazoa and Metazoa, reproduce sexually, the new individual arising by a fusion of two cells, one, the spermatozoon, being produced by the male, the other, the ovum or egg, by the female parent. These two reproductive cells always differ greatly in size and appearance but have an equal influence on the characters of the animal arising from their fusion.
The spermatozoon is a small, actively moving cell, which, in the majority of animals, has a very characteristic structure consisting of a head, middle piece and tail. The head is almost entirely oc cupied by the nucleus, capped by a body called the acrosome and surrounded by a thin layer of cytoplasm. The middle piece is cytoplasmic in nature, containing certain granules, of which the most important are a centrosome which is responsible for the first division of the fertilized egg, and a basal granule which controls the activities of the long tail, a thread of protoplasm, which by its spiral lashing drives the whole forward.
The ovum differs from the spermatozoon in being relatively large and immobile. It is a cell containing a nucleus and much cytoplasm which includes a food store of greater or lesser amount. This food reserve has to supply all the needs of the developing embryo until it becomes capable of feeding. As this event occurs at very different times, the amount of yolk, and hence the size of the egg, varies greatly in different animals.
Fertilization (q.v.) is the act of fusion of a spermatozoon with an egg. It is followed immediately by a change of egg-surface which renders it impossible for any more spermatozoa to enter its substance. The entry of a spermatozoon begins a series of activi ties whose result is the formation from the apparently simple and uniform egg of the elaborately differentiated body of the adult. The first stages of this process are similar in essentials in all Metazoa, but the courses of development of the individuals of the different phyla of the animal kingdom part company at stages depending on the remoteness of their relationship.
After the spermatozoon has entered the egg, usually leaving its tail outside, its head swells up and the fertilized egg contains two very similar nuclei. These approach one another and finally fuse. Meanwhile the centrosome brought in by the sperm has divided into two parts, which separate, a spindle forming between them.
The originally single egg then divides itself into two new cells, the blastomeres of the two-celled stage. These are usually, but not always, exactly similar in appearance. In the case of the sea urchin, Echinus esculentus, this division, which is called the first cleavage, takes place about one hour and ten minutes after fer tilization and occupies about half an hour.
Subsequently, in Echinus an hour later, each of these two blas tomeres divides into two, giving rise to a four-celled embryo. The plane of this second cleavage is usually at right angles to that of the first, so that normally the four blastomeres lie in a ring, in contact with one another.
Each of these cells then divides again by a third cleavage, often at right angles to the two which have preceded it. The cleavage of the four cells takes place simultaneously. This process of cleav age is continued until there is built up a mass of some 128 cells, a morula. The morula, if formed at all, typically resolves itself into a blastula in which the cells are arranged in a layer, one cell thick, surrounding a cavity, the segmentation cavity or blasto coel. The blastula is usually larger than the egg from which it has arisen because it has imbibed water from its surroundings.
The next process in development is known as gastrulation. It has as its object the formation of a primitive food cavity, the archenteron, surrounded by a double cellular wall, an inner layer of cells, the endoderm or hypoblast, which actually forms the wall of the gut and is concerned with digestion and the absorp tion of food, and an outer ectoderm or epiblast, which is the part of the embryo which comes into contact with the outside world.
In relatively small eggs this transformation is brought about by the actual intucking of one pole of the blastula into the other. In larger and more yolky eggs so simple a mode of formation is impracticable, and various divergences are found which may be regarded as modifications of the primitive method. Thus, in a frog's egg an actual intucking takes place, but is restricted to one point on the surface of the blastula, and leads to the formation of a shallow pit, subsequently lengthened by a very active localized growth of the part turned in, and a spread of organization from it forward into a mass of cells, which fills most of what should be the blastocoel.
In such eggs as that of a bird the endoderm is formed by the splitting off of a layer of cells from a comparatively thick mass formed by the cleavage divisions, by a process called delamination. The gastrula formed by any of these processes corresponds in its fundamental architecture with the adult animals which com pose the phylum Coelenterata (q.v.). In the development of the members of all higher phyla a third cell layer, the mesoderm, is established. This separates the ectoderm from the endoderm and from it are formed the majority, at any rate, of the muscles of the adult animal, its blood and blood vessels, and the gonads.
The mesoderm may arise in many different ways, it may be entirely derived from special blastomeres very early set apart in cleavage, it may come from the endoderm, or from the ectoderm.
In all the higher phyla the mesoderm either is divided into two sheets from its first appear ance, or becomes so later ; one of these sheets lines the ectoderm and the other encloses the endo derm of the gut. The cavity which separates these mesodermal sheets is the coelom or body cav ity. This space, which comes into existence to separate the gut from the body wall, is in most animals connected to the exterior by a canal or canals, the coelomoducts, whose primary function is to ex trude the reproductive cells from the coelom in whose wall they have been formed, and into which they have been set free. Subsequently, in the evolution of many forms, these coelomoducts become modified so as to subserve the function of excretion of nitrogenous waste products.
Primitively the coelom with its walls, the primordium of the mesoderm, seems to have arisen as a pouch pushed out from the archenteron, but in the majority of animals this mode of origin has been lost, the mesoderm arising from as few as two cells, early set aside ; from mesenchyme cells which have wandered out from the endoderm, or by "delamination" by being cut off as a sheet from the endoderm or even the ectoderm.
Subsequently a split, a schizocoel, appears in the originally solid mesoblast so formed. The embryo at this stage possesses a gut, the enteron, surrounded by the endoderm and communicating with the exterior by a single opening, the blastopore. The ectoderm forms the whole external surface, and from it arise the nervous system and sense organs ; in certain embryos arising from eggs which are shed into water, the ectoderm also produces cilia which serve for locomotion and feeding.
At this stage the blastopore often closes completely, only to reopen as the anus, the mouth being formed as a result of the perforation of an area of contact between the endoderm and the ectoderm. When the mouth and anus are established the embryo can begin to feed and passes into the larval stage.
The term larva (q.v.) properly implies an animal at a stage in its development when it can feed and move about without having acquired the structure of an adult. The larva may have a very short free life, a few minutes or hours in the case of some molluscs and tunicates, or it may cover the greater part of the individual's existence as in the case of the caterpillars.
The larva may be adapted for a mode of life and even to a medium entirely different from that of the adult animal which it will eventually become ; in which case the passage to the adult condition is usually carried out rapidly, by a process involving the destruction of the larval tissues by wandering phagocytic cells and the building up of their material into new adult tissues and organs. Such a transformation is known as a metamorphosis (q.v.). It is well illustrated by the mode in which a caterpillar becomes a pupa, all its tissues, except the nervous system and the gonads, becoming reduced to a structureless mass of cells, which are then built up into the body of the butterfly which will emerge from the chrysalis. The larval stage in a life history is thus designed to secure the possibility of growth, under favourable conditions, be tween the period when the food store originally laid down in the egg is exhausted and that when the adult form is reached.
In animals of very many groups the egg is fertilized within the body of the female which produced it (a process usually requir ing special intromittent structures in the male) and may be re tained there until development has gone on to some definite point. The new individuals are then expelled either as larvae or as small copies of the parent. This process is called viviparity.
In its early stages this condition implies no more than a reten tion of the egg inside the mother, but in its fully developed form it involves the provision of food for the growing embryo, either in a condition in which it can be eaten by what is essentially an unborn larva, as in certain fish, or by diffusion into the embryo from the mother's blood, usually through a special organ, the pla centa, as in mammals. Viviparity has the advantage that the few young that are born make their appearance at a larger size and far more advanced in structure than would otherwise be possible, and hence have better chances of survival.
It is obvious that a period of development is a necessity for a metazoan : only in such a way can the elaborate body of one of the higher animals be built up from a single cell. It is not, however, clear that such a development must follow so similar a course in all the higher forms. The only explanation is that all these animals are blood relations which owe the differences which sepa rate them to evolution, and that the courses of their development, originally identical, have come to differ by modification of the later stages, cleavage and gastrula remaining much the same in all.
The essentials of this conception were first stated by Carl Ernest von Baer in 1824. Much later Ernest Haekel made a more rigid generalization, his so-called biogenetic law, "that Ontogeny, the development of an individual animal, is a shortened recapitu lation of Phylogeny, the evolutionary history of the species to which it belongs." Haekel compared the fertilized egg to a proto zoon, the blastula to such a protozoan colony as V olvox, the gas trula to a coelenterate and so on. Belief in the general validity of Haekel's law led to an immense activity amongst zoologists in the last quarter of the 19th century. More and more refined methods of observation were elaborated and applied to representa tives of all the phyla, and special efforts were made to work out the development of the more primitive members of each group, with the hope, seldom justified by results, that knowledge of it would make plain their phylogenetic relationships. This work, although it can scarcely be said to have succeeded in its original aim, established morphology as a discipline with an accepted technique and theory. It made it certain that development would afford most valuable evidence about the homologies of organs, controlling conclusions reached by comparative anatomy.
The most interesting of the later developments of observational embryology was the study of "cell lineages," which led to the remarkable discovery that the early development of a mollusc followed a most elaborate course, individual cells and small groups of cells being very early set apart for the production of definite structures in the larva and adult. Even more unexpected is the identity of this process in molluscs and annelids.
It must have been realized from the beginning of the study of embryology that there is present in the fertilized egg a mechanism which controls the subsequent development, and the actual course of that development, in such forms as molluscs, suggests that it may be possible to discover its nature.
The only possible method for such an investigation is an experi mental one. The course of the development may be modifiable by altering the environment, the temperature, the chemical composi tion of the water in which it takes place, the space available for the growing embryo, and so on.
On the other hand materials may be removed from the egg or the embryo and any resulting defects noted. Two eggs may be caused to fuse and the history of the joint structure determined. The earliest stage of development, fertilization, may be demon strated by such methods to involve two separable processes. One of these, the modification of the egg surface and the beginning of cell division, can, in very many cases, be brought about by chemical means, or even by pricking with a glass needle smeared with blood serum. The other, the change in structure from the bringing in of paternal characters, can clearly not be copied artificially.
The embryo at the two-celled stage may be divided into two, and the subsequent history of the blastomeres traced. In different animals each may develop into a complete but half-sized larva, into a half larva, or one into a complete, the other an incomplete embryo.
Similar experiments may be performed on four- or eight-celled stages, or a blastula may be cut into pieces and their fate deter mined. The egg may be centrifuged so as to rearrange its con stituent parts, subsequently fertilized and allowed to develop, or it may be held upside down so that gravity will disturb the normal orientation of its materials.
From such experiments the conclusion has been reached that the cytoplasm of the egg possesses a structure which determines the course of development, and that it gains this structure before its maturation, whilst the oocyte is growing. Experiments have been designed and carried out with the intention of investigating the period in the development when cells become committed to the formation of certain definite structures or types of structures.
Thus, small pieces of the developing larva of a newt have been transplanted into new regions in corresponding embryos of the same or some other species. They may grow up there, forming part of a complete animal, exactly as if they had been originally formed at that spot. (See EXPERIMENTAL EMBRYOLOGY, INVERTE BRATE EMBRYOLOGY, VERTEBRATE EMBRYOLOGY.) (For embryology in plants, see: ANGIOSPERMS : Embryology.) (D. M. S. W.)