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 
quartet elements which are shown in fig. 79 surrounding the blastopore. 
The stomodeeal 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 stomodzeum 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 stomodzeal invagination has as yet not formed an open con- 
nection with the enteron, but shortly afterward this occurs, at which 
time the stomodzeum 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 Mollusks in which the blasto- 
pore closes and the stomodzeum forms at the same point. Among 
them may be named Nassa (Bobretzky), Neritina and Aplysia (Bloch- 
mann), Elysia (Vogt), various AZolidide (Trinchese), Doris (Langer- 
hans), Crepidula (Conklin), Planorbis (Holmes) and Trochus (Robert). 
In Patella (Patten), Fusus (Bobretzky), Pteropods and Heteropods 
(Fol) and Limnea (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 
