DIFFERENTIATION OF THE ORGANISM. 



19 



is a simple one, agrees in character, although it may be for a time 

 only, with many lower unicellular organisms (Protoplasta). 



The egg-cell undergoes changes, which ordinarily commence 

 after impregnation, and which are accompanied by changes in the 

 nucleus (the so-called germinal vesicle). In its place, and in part 

 from the material which formed it, two new nuclei arise, and 



Figs. 2-5. Various stages of the so-called cleavage process (Division of the egg). 



the egg-cell now begins to divide. Two cells thus arise, which 

 are either like one another, or differ from one another, in 

 size or in constitution. In both cases something fresh has arisen 

 from the egg-cell, and in both there is a differentiation, for 

 two parts have arisen from it. Four, eight, sixteen cells, and 

 so on, are formed by continued division, although of course 

 not always quite regularly, until at last a number of cells are 

 formed. This process of the division of the egg-cell is known as 

 the "segmentation of the yolk," and is a constant phssnomenon, 

 although it may present various modifications, which are always 

 due to adaptation, and which may be so explained. 



This is the first course of differentiation in the organism ; in 

 place of a single cell, a number of cells, similar to, or different from 

 one another, arise. The functions of the organism, which were 

 all performed previously by the egg-cell, are now performed by 

 the separate cells. The division of the egg-cell must therefore be 

 considered as leading to a division of its functions, although 

 indeed this division is merely a quantitative one. 



The various stages of this process of division have other relations 

 also, for they appear to agree in character with the mature stage 

 of many lower organisms (Protista), as for example the Volvo cineae 

 and Catallacta, in the developmental history of which there is at one 

 time an organism composed of a number of equi-formal cells. The 

 animal organism, therefore, even in the commencement of 

 its ontogeny, passes through several morphological stages, 

 which are permanent among the Protista, and the process 

 of segmentation of the ovum may be explained as a sur- 

 vival transmitted from early ancestors. Accordingly the 

 teleological halo, with which it would necessarily be sur- 

 rounded, were we limited to seeking its explanation exclusively 

 in connection with the future organism which is to arise from 

 this segmentation, is cleared away. The organism does not, how- 

 ever, get a specifically animal character from this formation of 



