CLEAVAGE AND DIFFERENTIATION. 35 



the gasteropods these resemblances of the cleavage stages are 

 marvelously accurate and complete; even among forms show- 

 ing the greatest diversity in the size and structure of the q.^%, 

 in the method of gastrulation and in the adult these resem- 

 blances are minute and long continued. Among the most 

 diverse types of prosobranchs, opisthobranchs, and pulmonates 

 very many blastomeres are identical in method of origin, rela- 

 tive position, and ultimate destiny. In fact, so far as now 

 known, all gasteropods have not only the same type of cleavage, 

 but all manifest the most fundamental similarity in the devel- 

 opmental history of individual blastomeres. 



The amount and distribution of yolk has little influence on 

 these resemblances. The Qgg of Crepidula adunca is 27 times 

 as large as that of C. plana, and yet every cleavage is identi- 

 cally the same up to the 52-cell stage. The ^^g of Fulgur is 

 140 times as large as that of C. plana, and yet the early cleav- 

 ages and the ultimate fate of the blastomeres is almost exactly 

 the same in the two cases. 



In the distribution of the yolk the most diverse conditions 

 are found associated with the most fundamental resemblances in 

 the origin and history of the blastomeres. In many eggs the 

 yolk is equally distributed to the first four cells, e.g., four 

 species of Crepidula, Neritina, Planorbis, Sycotypus, Fulgur, 

 and Bulla. In others it is chiefly aggregated in one, two, or 

 three of the macromeres, e.g., Urosalpinx, Illyonassa, Tritia, 

 Aplysia, Umbrella, etc. In general, if one macromere is larger 

 than another, it is the posterior one among prosobranchs and 

 the anterior one among opisthobranchs. Although this unequal 

 distribution of yolk makes marked changes in the form of the 

 embryo, it scarcely influences in a single respect the typical 

 formation and development of blastomeres. 



In one respect there seems to be a notable difference between 

 forms otherwise remarkably alike. In a large number of gas- 

 teropods (Neritina, Planorbis, Vermetus, Aplysia, Urosalpinx, 

 Tritia, Nassa, Illyonassa, etc.) the first and second cleavage 

 planes are oblique to the median plane of the embryo, whereas 

 in another series of forms (Crepidula, Umbrella, Sycotypus, 

 Fulgur, etc.) the first cleavage is approximately transverse to 



