612 



INVERTEBRATA 



CHAP. 



found, in all the divisions which occur during the development of the 

 egg asters are formed. As the zygote-nucleus moves inwards it 

 draws with it the clear cytoplasm, leaving most of the yellow cyto- 

 plasm behind. Some clear cytoplasm, however, is left at the surface 

 as a narrow band above the yellow crescent, and some yellow 

 cytoplasm moves inwards. The egg is now ready to undergo the 

 first cleavage. 



y.cr 



chn 



mes 



FIG. 443. Stages in the segmentation of the egg of Cynthia partita. (After Conklin.) 

 [The different coloured cytoplasms are indicated as in the preceding figure.] 



A, 2-cell stage viewed from behind. B, 8-cell stage viewed from the vegetative pole. C, IG-cell 

 stage viewed from the vegetative pole. D, segmenting egg passing into the 32-cell stage, viewed 

 from the vegetative pole, ch.n, chorda-neural cells ; ect, ectodermal cells ; ect.cy, cytoplasm which 

 will give rise to the ectoderm ; mes, mesodermal cell ; p.b, polar bodies ; v.p, vegetative pole ; y.cr, 

 yellow crescent. 



The first cleavage divides the egg into two precisely equal cells ; 

 the yellow crescent is also divided into two equal parts, and into each 

 crescent, after division, an intrusion of grey yolk takes place, which 

 indents it (Fig. 443, A). The two sides of the indentation join round 

 the intrusion, and the crescent is thus re-established. In this way 

 the yellow substance receives accessions from the yolk. After 

 division the nuclei, with the clear substance surrounding them, rotate 



