TEE SIZE OF ORGANISMS 183 



Boveri, Conklin, 5 et ah). 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 

 are 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 

 larvas 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 incon- 

 stant 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 lack- 

 ing 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 con- 

 stant number even for different species of a genus, especially if the eggs 



* Conklin, "The Organization and Cell Lineage of the Ascidian Egg," Jour. 

 Acad. Nat. Sci. Phila., 13, 1905. 



