THE GERM-CELLS 



123 



physiological division of labour ; and while it is no doubt the most 

 fundamental and important differentiation in the multicellular body, 

 it is nevertheless to be regarded as differing only in degree, not in 

 kind, from the distinctions between the various kinds of somatic cells. 

 In the lowest multicellular forms, such as l^olvox (Fig. 57), the 

 differentiation appears in a very clear form. Here the body consists 

 of a hollow sphere, the walls of which consist of two kinds of cells. 

 The very numerous smaller cells are devoted to the functions of nutri- 











Fig. 57. — Volvox, showing the small ciliated somatic cells and eight large germ-cells (drawn 

 from life by J. H. E.MERTON). 



tion and locomotion, and sooner or later die. A number, usually eight, 

 of larger cells are set aside as germ-cells, each of which by progressive 

 fission may form a new individual like the parent. In this case the 

 germ-cells are simply scattered about among the somatic cells, and no 

 special sexual organs exist. In all the higher types the germ-cells 

 are more or less definitely aggregated in groups, supported and nour- 

 ished by somatic cells specially set apart for that purpose and forming 

 distinct sexual organs, the ovaries and spenuarics or their equivalents. 

 Within these organs the germ-cells are carried, protected, and nour- 

 ished ; and here they undergo various differentiations to prepare 

 them for their future functions. 



In the earlier stages of embryological development the progenitors 

 of the germ-cells are exactly alike in the two sexes and are indistin- 



