TRANSFORMATION OF THE BLASTOPORE. 141 



by gradually narrowing, into the mouth. An ectodermal depression 

 does, indeed, regularly accompany this process, pushing the actual 

 blastopore some distance inward. Such a direct passage of the 

 blastopore into the mouth has been claimed by Bobretzky for Fusus 

 and by Fol for the Pteropoda and Heteropoda. The narrowing of 

 the blastopore may, further, lead to its direct closure, but even in 

 such cases, the stomodaeum forms at the same spot, as, for instance, 

 in Nassa and Neritina (Bobretzky and Blochmann). The point 

 at which the blastopore closes and where the adult mouth eventually 

 forms, no longer corresponds to the vegetative pole, i.e., to the end 

 of the embryo which is turned away from the animal pole, but, in 

 consequence of the growth of the postero-dorsal region, has shifted 

 somewhat towards the animal pole and is found behind the velum. 

 This last condition of the blastopore is that which is by far the most 

 frequent among the Gastropoda. Here also the blastopore is at first 

 round and may have a considerable diameter, but it soon becomes 

 narrow and slit-like (Planorbis, Patella, Palwlina and many other 

 < Jastropods). The slit-like blastopore closes from behind forward, 

 and its anterior end either passes direct into the mouth, as in Plan- 

 orbis, Linninea, and Patella (according to Eabl, Eay Lankester, 

 Wolfson, Patten) or closes completely, in which case, at the last 

 point to close, an ectodermal depression forms which yields the 

 stomodaeum. This latter is the case in Aplysia, Bythinia, and 

 Grepidula (Blochmann, Sarasin, v. Erlanger, Conklin). The 

 formation of the definitive mouth is always connected with an 

 invagination of the ectoderm. 



The slit-like blastopore, if regarded as open from its posterior to 

 its anterior end, seems to occupy the whole length of the later ventral 

 surface. Its posterior end no doubt still corresponds approximately 

 to the former vegetative pole, its anterior end lying immediately be- 

 hind the velum. Now while, in the majority of cases as yet known, 

 the blastopore closes from behind forward, in Paludina, as already 

 described, the posterior part of it is said to persist and to yield the 

 anus in the same way as the anterior part in the above cited cases 

 yielded the mouth. If this is actually the case, it can only be 

 explained by means of the view adopted by Butschli, according 

 to which both the mouth and anus arise by the differentiation of the 

 blastopore. Bctschli found an indication of this in Ray Lan- 

 kester's observations on Limnaea, in which form, the slit-like 

 blastopore, the anterior end of which becomes the mouth, extends as 

 far as to the anal region. Since that observation was made, other 



