XVII 



PEOTOCHOEDATA 



631 



exactly to what would have developed out of them had their sister 

 blastomeres remained alive. 



Conklin therefore, may fairly claim to have proved that these 

 coloured cytoplasms are definite organ-forming substances. He has 

 P'''^ n. *^*^s™ila^ substances exist in the eggs of Ciona, Molgula, 

 and Phallusia (1911), only that these substances are not distinguish- 

 able from one another in the living egg, but require staining to 

 ditierentiate them from one another. 



But now the question arises: if the organs of the larva are 

 determined by substances situated in definite regions of the unseg- 

 mented egg, whence do those substances come ? The answer to this 



mck 



p.tnch 



Fig. 458. — Illustrating the result of the development of single blastomeres of the 2-cell 

 stage of the egg of Gyiilhia partita. (After Conklin. ) 



A, frontal longitudinal section of young tadpole of Cynthia partita developed from normal egg. B, 

 frontal longitudinal section of young tadpole of Cynthia, partita developed from 2-cell stage in which 

 the left blastomere was killed, a.rixch, anterior mesenchyme (corresponding to the superior mesenchyme 

 of Fig. 446) ; ch, notochord ; ect, ectoderm ; nis, mesodermal hand ; pli, pharynx ; ip.mch, posterior 

 mesenchyme (corresponding to the inferior mesenchyme of Fig. 446). 



is, that one of them, the clear ectoderm-forming substance, is 

 actually contained in the nucleus. Further, Schaxel (1910) has shown 

 that, in the developing oocyte, chromatin is emitted from the nucleus, 

 and that the slaty-blue yolk spherules grow at the expense of these 

 chromatin particles. Hence we may boldly assert that the cytoplasm 

 of the ripe egg is differentiated by the egg-nucleus. 



But although the substances axe formed under the influence of the 

 egg-nucleus, they are arranged under the influence of the sper- 

 matozoon. As we have seen, the entering spermatozoon drags the 

 yellow substance after it, and thence determines the position of the 

 muscle-forming material. If two spermatozoa enter the egg, then the 

 yellow substance forms two aggregations instead of one. In the egg 



