Page:Popular Science Monthly Volume 83.djvu/187

Rh Boveri, Conklin, et al.). In the most highly differentiated cells {e. g., muscle, nerve) growth takes place independently of cell division; in less highly differentiated cells (e. g., epithelium, mesenchyme) the two processes go hand in hand.

It is an important fact that growth in size and growth in complexity arc separable processes, for although they are usually coincident during embryonic development they are not causally united. Just as growth in body size may, or may not, be accompanied by growth in complexity, so cell division may, or may not, be accompanied by differentiation. Cell divisions may thus be classified as differential and non-differential; the former are associated with growth in complexity as well as in size, the latter with growth in size only; the former are relatively constant in number for a given species, the latter vary in number with the size of the individual. The earlier cleavages of the egg are more generally differential than are the later ones, and within the same genus and even in related genera and phyla the number and character of differential cleavages is very constant. Thus in all annelids and mollusks, with the exception of cephalopods, the ectoderm comes from three quartets of cells which are cut off, one after another, at the animal pole of the egg, and in all cases each of these quartets gives rise to homologous regions of the larvæ of the different forms; the left posterior member of the fourth quartet (4d) is the mesentoblast and in all annelids and mollusks (except cephalopods) it gives rise to the mesodermal bands and to the posterior part of the intestine; and in general homologous portions of larval or adult animals come from homologous portions of the eggs of these animals through the medium of homologous differential cleavages.

On the other hand, non-differential cleavages are relatively inconstant in number, position and character; they vary greatly in number in different species, or even in different individuals of the same species, depending upon the size of the egg or embryo. Thus in different species of the genus Crepidula the differential cleavages are almost precisely the same in all, though the relative volumes of the eggs of different species vary from 1 to 27, but the non-differential cleavages are much more numerous in the large eggs than in the small ones. It is the fact that the earlier cleavages of eggs are so generally differential that makes possible the study of cell lineage; if such cleavages were generally non-differential they would be relatively inconstant and lacking in significance.

In animals with determinate cleavage of the egg the number and nature of the cells at any given stage of differentiation is, under normal conditions, absolutely constant for each species, and it may be a constant number even for different species of a genus, especially if the eggs