1896.] Embryology. 421 



elusions with regard to the foot. This, he says, is to be distinguished 

 very much earlier than has hitherto been recorded and, as one 

 would naturally expect from Patten's study of Patella, which he 

 quotes, it arises from a pair of folds and not from a single one as 

 has generally been stated for related forms. In Succwea these two 

 folds appear close behind the blastopore between the region of the 

 mouth and anus, and approaching one another fuse in the median line 

 forming an oval area. A median furrow persists for some time as evi- 

 dence of the union as in Patella. This last fact gives some meaning to 

 the similarly furrowed appearance occuring in Limnmis, Planorbis 

 and Ancylns. 



A study of over 100 embryo showed him that this paired origin is 

 the rule, although examples were found where the elevation was un- 

 paired, forming then a broad disc. In one apparently pathological 

 case the blastopore had retained its supposed primitive elongated form 

 and the beginning of the foot had the form of a horseshoe embracing 

 its hinder end. 



His conclusion that the foot represents the fused lips of the elon- 

 gated blastopore removes the possibility of the organ being some kind 

 of secondary formation, and makes it out to be a metamorphosed very 

 ancient structure: and if the conclusion is correct, the molluscan foot 

 is not quite such an anomalous structure as it has hitherto seemed. 



A few remarks concerning the podocyst and the so called " Nacken- 

 blase " are of interest in that they show that the latter structure is not 

 an organ at all, and that the contracting motions that have been ob- 

 served in it are due to the contractions of the podocyst which acts as 

 an organ of circulation. For in Sitcrinea where the structure in ques- 

 tion has an enormous development and where no podocyst occurs there 

 are no such movements to be seen. The structure is, he says, a mass 

 of endoderm cells swollen with albumen, the embryonal liver and the 

 outer body epithelium. 



With regard to the shell gland, Schmidt substantiates, in the main, 

 the early observations of Gegenbaur or Clausilia and shows Kor- 

 schelt's doubt concerning them to be unfounded. A large series of 

 Clausilia embryos gave ample opportunities for study, and as a result 

 it appears that very early the shell gland arises as an invagination of 

 the outer epithelium, and closing up, becomes completely cut off from 

 its parent layer. Sections show it to be completely surrounded by 

 mesoderm. The hollow vesicle thus formed becomes flattened out so 

 that he distinguishes in it an outer and an inner layer of cells sepa- 

 rated by a narrow space. The outer layer remains more or less un- 



