1904.] NATURAL SCIENCES OF PHILADELPHIA. 387 



may be observed upon lateral optical section dipping down from the 

 region of closure to the enteron beneath. These cells have come largely 

 from the third quartet of all four quadrants, and represent the smaller 

 cells of this quartet which lay nearest the open blastopore. This 

 condition exists but for a short time, for soon a broad pit may be ob- 

 served in this region occupying exactly the place where the blastopore 

 closed. As it forms the cells which have been invaginated to form the 

 blastopore-plug open out again so that a blind pit results, the lower 

 surface of which is formed by those cells which were first pushed inward 

 as the blastopore was closing, and correspond to the second and third 

 C[uartet elements w^hich are shown in fig. 79 surrounding the blastopore. 

 The stomodseal invagination continues to increase in depth by growth 

 and division of the cells which already form it and by further invagina- 

 tion of surrounding cells, so that, as the form of the veliger begins to ap- 

 pear (figs. 103, 104, 105, 106), second and third quartet cells from all 

 the quadrants lying in the region probably become involved. , At first 

 the stomodseum is broad and shallow, but as it increases in depth it 

 narrows and becomes more dorsally directed at its inner end. In 

 section, fig. 90, and in drawings of the oldest veliger shown (figs. 109, 

 110), the stomodseal invagination has as yet not formed an open con- 

 nection with the enteron, but shortly afterward this occurs, at which 

 time the stomodseum is much elongated. Union is established with the 

 stomach pouch just below the opening of the large liver lobe. 



Fiona agrees with a large number of JMollusks in which the blasto- 

 pore closes and the stomodseum forms at the same point. Among 

 them may be named Nassa (Bobretzky), Neritina and Aplysia (Bloch- 

 mann), Elysia (Vogt), various ^Eolididse (Trinchese), Doris (Langer- 

 hans), Crepidula (Conklin), Planorbis (Holmes) and Trochus (Robert). 

 In Patella (Patten), Fusus (Bobretzky), Pteropods and Heteropods 

 (Fol) and Limnwa (Lankester) the blastopore is said to remain open 

 and pass over directly into the mouth. 



Shell-gland and Foot. 



If one examines the segmenting egg somewhat later than such a 

 stage as shown in fig. 73, it will be observed that the posterior has con- 

 siderably outstripped the anterior region in extent and that, together 

 with numerous divisions, the cells have also enlarged considerably in 

 size. The area which lies along the median line, and so is derived from 

 the second quartet, shows most plainly this rapid increase in extent, 

 and it is here particularly that the cells themselves become greatly 



