GROWTH AND DIFFERENTIATION OF THE GERM-CELLS 1 1/ 



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

 surrounding the nucleus (Fig. 59, C, D) and advances thence towards 

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

 (e.g. in myriapods, Balbiani) it appears in irregular patches scat- 

 tered quite irregularly through the ovum (Fig. 59, A). In Branchi- 

 pns 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 egg cannot be explained as the result of a simple mechanical dis- 

 placement of the germinal vesicle by the yolk, as some authors have 

 maintained. Neither do they support the view that the actual polar- 

 ity of the egg exists from the beginning. They probably arise rather 

 through the varying physiological conditions under which the egg- 

 formation takes place ; but these have not yet been sufficiently 

 analyzed. 1 



The primary origin of the deutoplasm-grains is a question that 

 really involves the whole theory of cell-action and the relation of 

 nucleus and cytoplasm in metabolism. The evidence seems per- 

 fectly clear that in many cases the deutoplasm arises in situ in the 

 cytoplasm like the zymogen-granules in gland-cells. But there is 

 now a great body of evidence that seems to show with equal clear- 

 ness that a part of the egg-cytoplasm is directly or indirectly derived 

 from the nucleus. There is no question that a large part of the sub- 

 stance of the germinal vesicle is thrown out into the cytoplasm at the 

 time of maturation, as shown with especial clearness in the eggs of 

 amphibia, echinoderms, and some worms (e.g. in Nereis, Fig. 71). 

 A large number of observers have maintained that a similar giv- 

 ing off of solid nuclear substance occurs during the earlier stages of 

 growth ; and these observations are so numerous and some of them 

 are so careful, that it is impossible to doubt that this process really 

 takes place. The portions thus cast out of the nucleus have been 

 described by some authors as actual buds from the nucleus (Bloch- 

 mann, Scharff, Balbiani, etc.), as separate chromatin-rods (Van Bam- 

 beke, Erlanger), as portions of the chromatic network (Calkins), or as 

 nucleoli (Balbiani, Will, Ley dig). There is no evidence that such 

 eliminated nuclear materials directly give rise to deutoplasm-granules. 

 They would seem, rather, to have the value of food-matters or forma- 

 tive substances which are afterwards absorbed and elaborated by the 

 cytoplasm, the deutoplasm being a new deposit in the cytoplasmic 

 substance. It is, however, a matter of great interest that formed 

 nuclear elements should be given off into the cytoplasm, in view of 

 the general role of the nucleus as discussed in Chapter VII. 



1 Cf. p. 288. 



