152 



THE GERM-CELLS 



egg lies in the egg-tube just below a very large nurse-cell, which, 

 when fully developed, has an enormous branching nucleus as shown 

 in Fig. 163. In these two cases, again, the nurse-cell is character- 

 ized by the extraordinary development of its nucleus a fact which 

 points to an intimate relation between the nucleus and the metabolic 

 activity of the cell. 1 



In all these cases it is doubtful whether the nurse-cells are sister- 

 cells of the egg which have sacrificed their own development for the 

 sake of their companions, or whether they have had a distinct origin 

 from a very early period. That the former alternative is possible is 

 shown by the fact that such a sacrifice occurs in some animals after 

 the eggs have been laid. Thus in the earthworm, Lumbricus terres- 



Fig. 76. Egg and nurse-cell in the annelid, Ophryotrocha, [KoKSCHELT.] 

 A. Young stage, the nurse-cell () larger than the egg (o). B. Growth of the ovum. C. Late 

 stage, the nurse-cell degenerating. 



tris, several eggs are laid, but only one develops into an embryo, and 

 the latter devours the undeveloped eggs. A similar process occurs 

 in the marine gasteropods, where the eggs thus sacrificed may 

 undergo certain stages of development before their dissolution. 2 



(b) Differentiation of the Cytoplasm and Deposit of Dcutoplasm. - 

 In the very young ovum the cytoplasm is small in amount and free 

 from deutoplasm. As the egg enlarges, the cytoplasm increases 

 enormously, a process which involves both the growth of the pro- 

 toplasm and the formation of passive deutoplasm-bodies suspended 

 in the protoplasmic network. During the growth-period a peculiar 

 body known as the yolk-nucleus appears in the cytoplasm of many 

 ova, and this is probably concerned in some manner with the growth 



1 See p. 338. 2 See McMurrich, '96. 



