340 PROCEEDINGS OF THE ACADEMY OF [Jwly, 



adaptation to secure a rapid extension of the anterior half of the blasto- 

 derm over the yolk. During the growth of the invagination the area 

 of the blastoderm remains stationary, or actually grows smaller; as 

 soon as it flattens out there is a very rapid increase in the area of the 

 blastoderm (cf. figs. 20 and 21), especially of that portion lying anterior 

 to the first cleavage plane. Coincidently with this increase in area the 

 secondary macromere, iB, begins to move forward in the furrow 

 between the macromeres and the^ blastoderm rapidly extends for- 

 ward. The remarkable extent of the growth of this anterior portion 

 of the blastoderm may be seen by comparing the positions of the organ 

 bases in figs. 19-30. In the earlier figures the shell gland and the two 

 cerebral ganglia form the angles of an isosceles triangle, the base of 

 which is directed forw^ard (figs. 21 and 22) ; in later stages the ganglia 

 separate more widely and the triangle becomes ec|uilateral (figs. 23 

 and 24); still later the cerebral ganglia are separated so widely that 

 they lie in line with the shell gland (figs. 27 and 28) ; and finally the 

 cerebral ganglia again approach each other on the ventral side of the 

 embryo (figs. 29 and 30). The greatest growth of the blastoderm 

 takes place in the area between and in front of the cerebral ganglia, 

 in the very region of the apical invagination, and it seems reasonable to 

 suppose that the remarkable growth of this region is associated with 

 the formation and subsequent flattening out of this structure. 



iMcMurrich (1886) described at some length this invagination, and he 

 compared it with a similar formation observed by Blochmann (1883) 

 in Neritina, and by Sarasin (1882) in Bythinia. He says: "It seems 

 very strange that an invagination so well marked as it is in Neritina 

 and Fulgur should disappear and leave no trace of its existence, but so 

 it seems to do." His further conclusion that it would be found to 

 occur in most, if not all, of the prosobranch gasteropods has not been 

 justified by later studies. 



3. SHELL GLAND, SHELL AND MANTLE. 



The shell gland is one of the earliest and largest of the developing 

 organs, and is the one most instrumental in shaping the form of the 

 embryo. It appears as an aggregation of ectodermal cells in the 

 median plane posterior to the apical pole (figs. 21, 22). These cells, 

 which are probably derived from the cell 2d, increase greatly in nimiber 

 and form a saucer-shaped depression (figs. 23-26, Sh.). This depres- 

 sion then becomes deeper and smaller in surface area (figs. 27, 28) and 

 subsequently it evaginates in the manner characteristic of gasteropods, 

 the margin of the gland forming a ridge, the mantle edge, while the 



