GROWTH AND DIFFERENTIATION OF THE GERM-CELLS 153 



of the cytoplasm and the formation of the yolk. Both its origin and 

 its physiological role are, however, still involved in doubt. 



The deutoplasm first appears, while the eggs are still very small, 

 in the form of granules which seem to have at first no constant posi- 

 tion with reference to the egg-nucleus, even in the same species. 

 Thus Jordan ('93) states that in the newt {Diemyctyhis) the yolk may 

 be first formed at one side of the Q.gg and afterward spread to other 

 parts, or it may appear in more or less irregular separate patches 

 which finally form an irregular ring about the nucleus, which at this 

 period has an approximately central position. In some Amphibia 



Fig. 77. — Ovarian eggs of insects. fKok.^cn Ki.i .J 

 A. Egg of the butterfly, Vanessa, surrounded by its follicle; above, three nurse-cells (//.c.) with 

 branching nuclei; g.v. germinal vesicle. B. Egg of water-beetle, Dytiscus, living; the egg (o.v.) 

 lies between two groups of nutritive cells; the germinal vesicle sends amoeboid processes into the 

 dark mass of food-granules. 



the deutoplasm appears near the periphery and advances inward 

 toward the nucleus. More commonly it first appears in a zone 

 surrounding the nucleus (Fig. jS, C, D) and advances thence toward 

 the periphery (trout, Henneguy ; cephalopods, Ussow). In still others 

 {e.g. in myriapods, Balbiani) it appears in irregular patches scattered 

 quite irregularly through the ovum (Fig. 78, A). In Branchipiis the 

 yolk is laid down at the centre of the egg, while the nucleus lies at 

 the extreme periphery (Brauer). These variations show in general 

 no definite relation to the ultimate arrangement — a fact which 

 proves that the eccentricity of the nucleus and the polarity of the 



