UNICELLULAR DEVELOPMENT. At the outset it may be well to consider a case of development in the Protozoa, or unicellular animals, in order that we may learn to distinguish between the essential and the secondary in the process. We may take as a type Podophyra, one of the Sue toria. Podophyra produces germs of new indi viduals by the nipping off from the cell-body of a spherical mass, including a nucleus. At the time that it is set free the germ possesses a girdle of cilia by means of which it swims about. It grows, i.e. increases in size. Finally it be comes attached at a point on the equator: be comes elongated. forming a stalk; develops suck ing tentacles at the upper extremity of the body, and becomes au adult Podophyra. It is to be lulled that: (1) The germ grows, and of course witlemt any cell division occurring,. (2) On the it becomes more complex as development proceeds: it is differentiated. (3) During devel opment the embryo assumes various activities for which its organs fit it; it is adapted at every stage to meet the environmental conditions of that stage. differentiation, and constant adaptation to environmental eonditions seem to be the essential charneters of development.
uria DEVEI OPMENT. in the Aleta zon, in which the body is multieellular, the process of eell-division has to he added to the developmental processes, but. this may be re garded as a concomitant of the great. size of the which demends nuclear material in every part. Dere the germ-cells may take the form either of an egg or of a spermatozoIIn.
EvoixTiox or TIIE EGo. The egg, or ovum, is a stimulus to or 'animated' the egg. and it was long thought that the fluid of the sperm and not the spermatozoOn Was the 'fertilizing' agent; the spermatozoa. indeed, were regarded as parasites.
It is now known that the essence of fertilization is the union of two cells. and probably especially the union of their nuclei; for the head of the spermatozoon is composed almost wholly of the nucleus, and it is especially the head that pen etrates into the egg. The head is at first very small compared with the nucleus of the egg-cell, but it rapid ly swells with water as it enters the egg, and when the nuclei unite they are of about the same size. The two nuclei contain the same number of chromosomes (see CELL ) , so that when they unite the number of chromosomes is doubled in the nucleus, half being derived from the mother and half from the father. Thus ma ternal and paternal qualities are united in the fertilized egg. and are united in equal amounts. This accounts for the equality of inheritance by the offspring from the two parents. In plants the fusion of nuclei derived from the ovule and from the pollen-grain occurs in a fashion exactly comparable with that of animals. The pollen grain typically contains two nuclei, of which the second has been seen in some cases to unite with a second nucleus of the ovule—namely, the endosperm nucleus. As a result of this union those structures of the seed that are derived from the endosperm seem to derive their qualities from the two parents. This result is most striking when the parents belong to different species, char acterized by different kinds of endosperm.