172 CONK LIN. [Vol. X 1 1 1 . 



line, while 5C lies on the dorsal side, Fig. 68. At the same 

 time all structures on the ventral side are carried to the right, 

 e.g., the fundament of the intestine, while structures on the 

 dorsal side are displaced to the left, e.g., the fundament of the 

 shell gland, Fig. 74. The ectoderm and mesoderm seem to 

 follow the entoderm cells in this torsion, as if they were being 

 passively shifted by the movements of the latter. 



In later stages, with the evagination of the shell gland, Fig, 

 'J?), the posterior end, morphologically, is shoved farther and 

 farther toward the ventral side. By the latter movement the 

 distal end of the intestine is carried forward on the ventral side, 

 by the laeotropic torsion it is moved up on to the right side of 

 the embryo. Figs. 80 and 81, until the alimentary canal crosses 

 itself like a figure 8 open at the top. These two motions com- 

 bined bring about the complicated form of asymmetry charac- 

 teristic of the adult. 



With the evagination of the shell gland, the yolk cells pro- 

 trude like an immense hernia through the lips of the gland, 

 being covered, however, by an exceedingly thin layer of large 

 ectoderm cells ; this portion of the embryo becomes the visceral 

 mass {Eingeweidesack). The point at which the shell gland 

 was first formed marks the summit of the spire of the adult 

 shell, and the lips of the invagination become the mantle edge, 

 as is well known. 



It is abundantly evident from this account that the asymme- 

 try of the adult Crepidula is not produced by the asymmetrical 

 development of the shell gland, as is usually maintained for 

 gasteropods in general. In fact the initial asymmetry of the 

 shell gland is produced by the torsion of the posterior end of 

 the embryo. In Crepidula the first recognizable cause of the 

 torsion lies in the asymmetry of the cells 5C and 5D. That 

 this has any profound phylogenetic significance, however, seems 

 to me rather doubtful. The yolk cells, because of their great 

 size, exercise an undue amount of influence upon the shape 

 of the entire embryo. It seems to me that phylogenetically 

 neither the yolk cells nor the shell gland were the source of 

 the torsion; they merely took part in a general twisting of the 

 entire posterior end of the embryo. 



