668 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



I ' AJiOjS'o* 1 O 



Fig. 217. — Ovarian tubule of 

 daphnid, Sida crystallina, show- 

 ing stage in genesis of parthe- 

 nogenetic egg. In each group of 

 four cells, g, the third from the 

 apical end of the ovary becomes 

 an oocyte, 0; the others are nu- 

 tritive cells (from Weismann, 

 i877,reproduced in Child, 19156) 



crystallina, as described by Weismann (1877), 

 the primitive germ cells of the apical ovarian 

 region become successive series of four cells 

 each, the third cell from the apical end in each 

 becoming oocyte, the others nutritive cells 

 (Fig. 217). How differentiation of oocyte and 

 nutritive cells is determined does not appear. 

 The larger winter egg, containing more yolk 

 when full grown, uses several series of cells in 

 its growth, sometimes as many as twelve. In 

 Polyphemus the relation is different. All nu- 

 tritive cells associated with a single oocyte are 

 at one pole of the latter, but this may be either 

 pole as regards relation to the ovarian tubule. 

 Polar-body formation at the pole without nu- 

 tritive cells indicates that the other pole be- 

 comes basal whatever its position with respect 

 to the ovary (Kiihn, 1912). 



In many insects oocytes and nutritive cells 

 develop from the primitive ovarian cells, all 

 apparently alike in early stages, with great 

 variation in number of nutritive cells associ- 

 ated with each oocyte. In other species an 

 epithelial follicle is in direct contact with the 

 entire surface of the oocyte, other nutritive 

 cells being absent. For convenience in descrip- 

 tion the end of the insect ovarian tubule con- 

 taining the primitive germ cells is regarded 

 as apical, the other end as basal. From early 

 stages cells in the ovarian tubule of the earwig 

 Forficula are associated in pairs (Korschelt, 

 1 891). The basal cell of each pair becomes 

 oocyte; the other, nutritive cell, develops a 

 large branched nucleus (Fig. 218, .4). The re- 

 lation between oocyte and nutritive cells in an 

 aphid, Melanoxanthum, is shown for the par- 

 thenogenetic egg in Figure 218, B, and for the 

 winter egg, requiring fertilization, in Figure 

 218, C. A protoplasmic strand connects oocyte 



