No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 391 



quadrants the smaller cell is nearer the animal pole, while on 

 the posterior side of the Qgg the reverse is the case. Which 

 of the cells are the first to divide is not stated. In another 

 pulmonate, Siphonaria, the division of the posterior cells of 

 the third quartette occurs before that of the anterior ones, 

 each cell giving off a small cell toward the vegetal pole. The 

 division in both cells is laeotropic, but less so in the cell on the 

 left side (Fujita, '95, p. 91, Fig. 5, 6x). This cleavage, accord- 

 ing to Fujita, occurs at the thirty-two-cell stage; the corre- 

 sponding cells in the anterior quadrant have not divided at the 

 stage in which the egg contains forty-three cells, which is as 

 far as the cell lineage of this form is described. 



The first division of the cells of the third quartette seems to 

 mark the point in the cleavage of gasteropods where bilateral 

 cleavage makes its first uncertain appearance. In no case is 

 there a typical spiral cleavage of all the four cells. The cleav- 

 age may be radial (Physa, Limax, Planorbis, Neritina, Umbrella), 

 slightly bilateral in the posterior quadrants, and spiral in the 

 anterior ones (Crepidula), or spiral in the posterior quadrants, 

 but with an approach toward the bilateral type (Siphonaria). 

 The posterior quadrants generally divide before the anterior 

 ones. In Limax, however, the order of cleavage in the differ- 

 ent quadrants seems to be inconstant (Kofoid). In Crepidula, 

 Umbrella, and Planorbis the posterior cells divide only a short 

 time before the anterior ones, while there is a long interval 

 between these divisions in Siphonaria. The cleavage of the 

 cells of this quartette is, in all the above forms, unequal, and, 

 except in the anterior quadrants in Physa, the smaller cell lies 

 nearer the vegetal pole of the Q:g^. 



The Formatiojt of the Primary Mesoblasts. — Soon after the 

 twenty-four-cell stage is reached, the posterior macromere D 

 divides into unequal parts. The upper moiety is much larger 

 than the lower one, and, from the first, lies partly pushed into 

 the cleavage cavity, so that only a small portion of it appears 

 at the surface of the egg (PI. XVII, Fig. 11). It is dark and 

 granular, and contains a large amount of yolk, like the cells of 

 the entoderm. The division of D is dexiotropic, as Crampton 

 found it to be in Physa, The primary mesomere lies, therefore. 



