71 
form a pair of pouches or diverticula, the halves of the liver, l f 
in the walls of which numerous highly refractive oil-globules 
make their appearance. 
Our knowledge of the digestive tract in the Mollusca is at 
present in the greatest contusion ; and in most molluscs the 
difficulties of observation are so great at the time when the 
most important changes take place, that it is almost impossi- 
ble to obtain any perfectly satisfactory results. 
The oyster presents exceptionally favorable conditions for 
investigating this question, and the observations above de*» 
scribed seem to show conclusively — 
1st. That there is an invaginate gastrula stage. 
2d. That the blastopore closes completely, leaving the diges- 
tive tract without an opening. 
3d. That the shell appears at the point which the blastopore 
previously occupied. 
4th. That first one and then two openings from the outer 
surface into the digestive tract make their appearance almost 
directly opposite the position of the blastopore, and that one of 
them becomes the mouth and one the anus. 
The most thorough and satisfactory account of the origin of 
the digestive tract in Lamellibranchs is that of Fabl (Ent- 
wickelung der Malermuschel), and the process, as he describes 
it, is like what 1 have observed in the oyster, so far as all the 
leading points are concerned. At the close of segmentation 
the single large macromere of TJnio divides into a number of 
large cells, which cover the dorsal surface of the embryo, Taf. 
X, Fig. 23. They then push into the body cavity, so as to 
form a primitive digestive cavity, which opens by a dorsal 
blastopore. Taf, XI, Fig. 28. 
The shell now appears, and covers up the dorsal surface,- 
Taf. XI, Fig. 34, and the .blastopore closes up, thus leaving the 
digestive tract without any opening. Taf. XII, Fig. 54. 
A new opening is now formed by the invagination of the 
integument, at a point on the ventral surface, Figure 54, 
which is at some distance from the blastopore, but not directly 
opposite it, as in the oyster. 
