GROWTH AND DIFFERENTIATION OF THE GERM-CELLS 



151 



a follicle, but in some forms each egg is accompanied by a single 

 nurse-cell, attached to its side, with which it floats free in the body- 

 cavity. In OphryotrocJia, where it has been carefully described by 

 Korschelt, the nurse-cell is at first much larger than the Qg^ itself, 

 and contains a large, irregular nucleus, rich in chromatin (Fig. j6). 

 The egg-cell rapidly grows, apparently at the expense of the nurse- 

 cell, which becomes reduced to a mere rudiment attached to one side 

 of the egg and finally disappears. There can hardly be a doubt, 

 as Korschelt maintains, that the nurse-cell is in some manner con- 

 nected with the elaboration of food for the growing egg-cell ; and 

 the intensely chromatic 

 character of the nucleus is 

 well worthy of note in this 

 connection. Still more in- 

 teresting are the conditions 

 observed by Wheeler ('96, 

 '97) in Myzostoma, where 

 the young ^^^ is accom- 

 panied by two nurse-cells, 

 one at either end. These 

 cells fuse bodily with the 

 &^g, one having " some- 

 thing to do in forming the 

 vacuolated cytoplasm at 

 the animal pole, . . . the 

 other in forming the granu- 

 lar cytoplasm at the vege- 

 tative pole" ('97, p. 42). 

 The polar axis thus deter- 

 mined persists as that of 

 the ripe ovum. This 

 seems one of the clearest 



cases showing the establishment of the egg-polarity through the 

 relation of the o,^^ to its environment.^ 



Somewhat similar nurse-cells occur in the insects, where they have 

 been carefully described by Korschelt. The eggs here lie in a series 

 in the ovarian " egg-tubes " alternating with nutritive cells vari- 

 ously arranged in different cases. In the butterfly Vanessa, each 

 ^g^, is surrounded by a regular follicular layer of cells, a few of 

 which at one end are differentiated into nurse-cells. These cells 

 are very large and have huge amoeboid nuclei, rich in chromatin 

 (Fig. Tj, A). In the ear-wig, Forficnla, the arrangement is still more 

 remarkable, and recalls that occurring in OpJuyotrocJia. Here each 



1 Cf. p. 386. 



Fig. 75. — Ovarian eggs of Helix. [Obst.] 



A. Earlier stage, surrounded by follicle. B. Later 

 stage, showing inward migration and absorption of fol- 

 licle-cells. 



