346 PROCEEDINGS OF THE ACADEMY OF [Jl-^b^ 



From the preceding account it is evident that during the later stages 

 of its development the vehim in Fulgur is much the same as in 

 Crepidula or any other prosobranch. In its early history, however, it 

 seems to be altogether different. It appears, as has been said, on the 

 median and posterior side of the cerebral ganglia when these structures 

 lie far apart in the posterior margin of the blastoderm (fig. 28) ; in 

 Crepidtda it first appears about the time of the closure of the blasto- 

 pore as a transverse row of cells in front of the mouth. In Fulgur it 

 consists of two bands of cells on opposite sides of the embryo, which 

 are entirely separate from each other ; in Crepidula it is a single struc- 

 ture continuous from side to side. 



However, in spite of these striking differences, it can be shown, I 

 think, that the velum has a similar origin in both of these genera. 

 Traces of the velum in Fidgur may be foimd at a much earlier stage 

 than that shown in fig. 28; thus in fig. 23 a row of what I take to be 

 velar cells may be seen on the lateral and anterior sides of the cerebral 

 ganglia (the nuclei of these cells are shown in heavy outline). The 

 position of this row of cells indicates that it has arisen from the cells 

 adjoining the ganglion on the anterior and lateral sides, and since the 

 ganglia themselves come, in all probability, from the "rosette" 

 cells, the velar cells must have come from the anterior "turret" cells 

 and perhaps also from the terminal cells of the transverse arms of the 

 cross. In short, the velar cells arise in Fidgur in the same region and 

 probably from the same cells as in Crepidula. Here again, as in the 

 case of the cerebral ganglia, there is fundamental agreement between 

 Fulgur and Crepidtda in the early and late stages in the developm.ent 

 of the velum; it is only in those stages of the overgrowth of the yolk, 

 which are undoubtedly highly modified in Fidgur, that we find striking 

 differences between these two genera. 



6. BLASTOPORE, MOUTH AND (ESOPHAGUS. 



By the very great growth of the blast oderin in the region of the 

 apical invagination the anterior portion of the blastoderm rapidly 

 surrounds the yolk, while its posterior margin, containing all the organ 

 bases, remains relatively fixed in position near the animal pole (figs. 

 25-28). Subsequently this posterior margin also extends over the 

 yolk to such an extent that the blastopore finally lies at the vegetal 

 pole of the egg (fig. 30). The anterior lip of the blastopore is always 

 composed of an extremely thin layer of cells, and this portion of the 

 blastopore is usually circular in outline; the posterior lip of the 

 blastopore is always composed of a thicker layer of cells than the an- 



