212 Edivni G. Conkliu. 



front of the yellow crescent and its anterior portion becomes the 

 gray crescent of chorda-neuroplasm, while its posterior portion is 

 the deep gray endoplasm which gives rise to the gastral endoderm. 



The form of localization of these substances, therefore, undergoes 

 marked changes during the fertilization and first cleavage; it is 

 concentric in the oocyte, polar or radial immediately after the 

 entrance of the sperm, bilateral just before the first cleavage, and 

 definitive at the close of the first cleavage. 



I have elsewhere ('05^) shown reason for believing that even in 

 the stage of radial localization in the egg of Cynthia there is prob- 

 ably some structural peculiarity of the egg which determines that 

 the path of the sperm shall lie in one meridian rather than in 

 another and therefore that the median plane of the embryo and its 

 posterior pole are not determined by the chance movements of the 

 sperm within the egg. Similarly the basis for polar or radial 

 localization is present in the ovarian egg in the slight eccentricity 

 of the germinal vesicle toward the animal pole, though the ooplas- 

 mic substances are largely localized in concentric form at this 

 stage. I am unable to determine whether any structural basis for 

 bilateral localization exists in the ovarian eggs of ascidians, but 

 inasmuch as the localization invariably becomes bilateral at a 

 later stage it seems necessary to suppose that there is some such 

 intrinsic determinative factor. 



In almost every group of animals the chief axis of the egg is 

 already marked out in the oocyte, the pole toward which the ger- 

 minal vesicle is eccentric becoming later the animal pole of the egg 

 and the ectodermal pole of the embryo. Despite this eccen- 

 tricity of the germinal vesicle the localization of ooplasmic sub- 

 stances in the oocyte of ctenophores, nemertines, echinoderms and 

 ascidians is chiefly concentric, the polar localization of these 

 substances first appearing during the maturation and fertilization. 

 On the other hand Wilson ('04) has found a markedly polar 

 localization of the ooplasm in the oocyte of Dentalium; while it is 

 probable that in the oocytes of insects and cephalopods the local- 

 ization is bilateral in form. 



Boveri ('01) found that distortion of the egg of Strongylocen- 

 trotus after the formation of the equatorial zone produced no 

 change in the polar stratification of the egg nor in the potencies of 

 its different substances. Wilson ('03), Yatsu ('04) and Zeleny 

 ('04) have discovered that fragments from any part of the egg of 



