.Nos. 451-452.] AMERICAN SOCIETY OE ZOOLOGISTS. 



as in the fate of the corresponding cells with the co 

 obtaining among the Polychreta, speaks str(iii,<;i\ tni- a 

 relationship between this group and the 1 )in(iii!iili(la 

 cleavage patterns are inheritable, as well as oilu r rh; 



Organ-formin^^r Gcnn Regions in the of 

 Snails: lulwin Cz. Conklin, University of \\^\^^^^^\^^^^\ 

 early development of ascidians and snail>. ni Muh 

 minate character, even in the earliest eKM\a,L;e Ma-rs. 



rials exists in the unsegmented egg. /\ study (<t the t 

 these animals shows that this antecedent i)n)bal)ility is 

 in fact. 



In the living eggs of the fresh water snails, IJmnaa, 

 and Planorbis, one may readily observe the segregation of 

 materials of the egg before cleavage begins. Before mati 

 the nucleus can be seen in such eggs as a large, clear 

 while the rest of the egg substance is filled with yolk g" 

 and is yellow in color. During the maturation divisio - 

 nuclear vesicle is dissolved and its contents form a cle 

 surrounding the polar amphiasters. After the secont 

 body is formed this clear area spreads out as a disk and 

 a cap which covers the upper two-thirds of the egg. Th- 

 third of the egg which is not covered by this caj) of clear 

 plasm remains of a bright yellow color. The^^e two ] 



clear area giving rise to the ectoderm and the yelh.w r.nc 

 endoderm and mesoderm. In these snails, iherch • ■ 

 layers are visibly outlined in the unsegmented egg. 



A somewhat similar localization of germinal mateiials. 



{Stye la) partita, Ciona intestinalis and Molgula man fir 

 the central yf)lk mass of the immature egg i^ >urrounde 

 sides by a peripheral layer of protoplasm. In the hvii 

 of Cynthia the central yolk is gray in color while the i.e 

 layer contains granules of yellow pigment ; in C iona the 

 red, the peripheral layer clear and transparent ; in Mol- 

 yolk is gray, the peripheral layer clear. As soon as a : 



