Vertebrate Embryology

In the ordinary Mammalia the early stages of development are, as has already been indicated, greatly modified. The modification is associated with two main causative factors: (I) the loss of the yolk, which is present in the more archaic vertebrates and (2) the development of the egg in a strictly confined space, owing to the presence of the shell-like tightly-fitting zona pellucida, followed, in some mammals, by being imbedded in the substance of the uterine wall. In the relatively primitive Indian tree-shrew Tupaia, this confinement of the blastula leads the apical part of its wall, where growth is most active, to dip down for a time into the cavity, and it would appear that this temporary involution of the apical pole in Tupaia gives the clue to one of the most puzzling peculiarities in the early development of the typical Mammalia. In such mammals segmentation results in a solid sphere of cells, into the interior of which fluid is secreted by the activity of the outer layer to produce a thin-walled blastocyst, distended with fluid and carrying at its apical pole, projecting into the cavity, an "inner mass" of cells, which later on flattens out and constitutes the most important formative portion of the blastocyst wall. The appearances in Tupaia clearly suggest that the inner mass of the ordinary mammal simply represents the actively growing apical portion of the blastocyst wall which had to find space for itself by bulging into the interior of the blastocyst, owing to the blasto cyst being unable to expand as a whole.

Mesoderm.

Gastrulation leads to the establishment of the two primary cell-layers, ectoderm and endoderm, but in the verte brate as in other coelomate animals, these constitute but a small fraction of the total mass of the body: by far the greater part is mesodermal in nature. In Amphioxus the mesoderm is repre sented for a time by a series of coelenteric pouches of endoderm down each side of the body. Each of these becomes isolated, so that they form a series of closed compartments on each side. These are the primitive mesoderm segments, and their cavities are the coelomic compartments. At this stage the mesoderm of Am phioxus is comparable in certain ways with that of an annelid worm, but in its further development a striking and highly char acteristic difference makes its appearance. In an annelid the coelomic compartments becoming distended with coelomic fluid form the wide body-cavity: their outer and inner walls provide the muscular layer of body-wall and enteric wall respectively: their headward and tailward walls applied to those of their neigh bours form the coelomic septa : the portions of their mesial walls, where not separated by interposed alimentary canal, form the dorsal and ventral mesenteries.

The difference seen in Amphioxus and in other vertebrates is that each mesoderm segment becomes divided into a dorsal and a ventral portion, which differ markedly in their further devel opment. The dorsal, separated from the corresponding structures of the other side of the body by the interposed spinal cord and notochord, becomes a muscle segment or myotome: its portion of coelome, the myocoele, becomes obliterated by the apposition of inner and outer walls. The ventral portion of the mesoderm

segment occupies the space between the endoderm and the body wall : its portion of coelome forms the splanchnocoele or body cavity. This ventral portion of the mesoderm—the so-called lateral mesoderm of typical vertebrates—develops two striking differences from the dorsal portion: it loses entirely its original segmentation, the splanchnocoele becoming continuous from end to end, and its outer wall no longer produces muscle Thus while in the annelid the whole extent of the lateral wall of the body is provided directly with a lining of muscle, in Amphioxus this ap plies only to the dorsal portion. With further development how ever the myotome extends in a ventral direction, insinuating itself between the splanchnocoele and the ectoderm, and in this way the ventral portion of the body-wall in Amphioxus, and other verte brates, becomes secondarily muscularized.

In the Amniota where, in correlation with the thinness of the blastoderm, the mesoderm segment is flattened out into a sheet of cells, this mesial or paraxial source of mesoderm becomes ap parently the chief one, and the mesoderm presents the appearance of growing out from the primitive streak region.

Commonly included with the typical mesoderm is the mesen chyme. This is a collective term to embrace cells which, assum ing an amoeboid character, creep away from their point of origin in the embryo, wandering through its body, multiplying by fission and behaving as if they were independent organisms. Some of these retain their amoeboid wandering character and constitute the leucocytes : others become erythrocytes or red blood-cor puscles: others become chromatophores: others settle down and form the packing tissue or connective tissue which forms the general framework of the body.

The Skin.

The skin of vertebrates is formed by the ectoderm, with a backing of tough connective tissue traversed in all direc tions by fine fibres and constituting the dermis. In all members of the group above Amphioxus the epidermis, by cell-multiplica tion, loses its original condition of being only one cell thick. In fish numerous epidermal cells become glandular and secrete slime. From the lungfish upwards, local aggregations of these cells form definite flask-shaped glands, opening by minute pores on the body surface. These epidermal glands undergo specialization in various directions—salivary glands, poison glands, sweat glands, milk glands, etc. In terrestrial vertebrates the superficial ectoderm cells become converted into keratin, forming a horny layer which obstructs evaporation, and these horny cells are shed from time to time as loose scurf, as coherent flakes, or as a continuous slough (e.g., snakes). In reptiles they form a hard layer, covering underlying bony plates in tortoise-shell or the surface of the head in reptiles generally, or forming, with a dense backing of connec tive tissue, the scales of ordinary reptiles. Innumerable special developments of the horny layer occur, some of which will b e found described in other articles : such are claws, feathers, hair, hoofs, rhinoceros horn, whalebone, etc.