350 EDWIN G. CONKLIN 



may be changed experimentally without changing the real cell 

 axis. In short the cell polarity persists in the organization of 

 the cytoplasm after the positions of centrospheres, nuclei, mi- 

 totic figures and cleavage planes have been changed. But while 

 in such cases the polarity of the cell persists in the cytoplasm, 

 there is evidence that the polarity of the cytoplasm has devel- 

 oped in connection with and in definite relation to the polarity 

 of the nucleus and centrosome. 



With regard to symmetry it is known in a few cases, notably 

 cephalopods and insects, that the egg is bilateral even in the 

 ovary; in other cases such as amphibians and ascidians, bilater- 

 ality first becomes apparent shortly after fertilization; in still 

 other cases bilaterality does not become evident until later stages 

 of the cleavage or even of the blastula or the gastrula. In the 

 case of sinistral gastropods the inverse symmetry may be 

 traced back in development to inversely symmetrical cleavage 

 of the egg, indeed to the very first cleavage, and it is evident 

 that the causes of this inversion must be present in the egg 

 before cleavage begins (Conklin '03). 



In addition to these general axial differentiations of polarity 

 and symmetry other more specific differentiations of regions and 

 substances of the egg exist in some animals. In ascidian eggs 

 the substances which give rise to different organs and tissues, 

 such as the nervous system, the chorda, the caudal muscles, the 

 mesenchyme, the ectodermal and endodermal epithelium are 

 definitely localized and may be clearly distinguished as early as 

 the first cleavage (Conklin ('05). Although these eggs show an 

 unusual degree of differentiation at a very early stage, there are 

 many others in which the 'pattern of localization' is present 

 either before or just after cleavage begins. Among the gastro- 

 pods generally the first cleavage separates an anterior blasto- 

 mere (A B) from a posterior one (C D), the second cleavage 

 divides these into right and left halves. Each of these four 

 blastomeres gives rise to three ectomeres and to a large ento- 

 mere, while the left posterior cell (D) gives rise also to the meso- 

 mere {Jfd) . Each of these cells produces in later stages a definite 

 portion of the embryo so that the development is, as Wilson has 



