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Brooks.] 328 [November 19, 
In many particulars the history of the digestive tract of the oyster 
is quite different from its history in the Pulmonates, although there 
are also many points of resemblance. 
The following are the leading points which have been made out in 
the oyster : | 
1. Segmentation takes place in substantially the manner described 
in various Lamellibranchs by Lovén, Rabl, and more minutely by 
Flemming, and there seems to be ample evidence to establish this as 
the normal form of segmentation in the whole class. It is peculiarin 
this, that although the egg is very small and contains very little 
deuteroplasm, the greater part of the yolk has the morphological with- 
out the physiological characteristics of a food yolk. Segmentation is 
restricted to the formative pole, and gives rise to a layer of cells which 
spreads down around the single larger spherule at the formative pole; 
this represents the food yolk, so far as its relation to the process 
of segmentation goes, although it contains a single nucleus, and 
no more food material than is contained by the segmenting portion or 
the egg. A comparison of Flemming’s figures of the process of seg- 
mentation in the very small egg of Anodonta, with the figures which 
Brobetsky gives of the same stages in certain prosobranchiate Gastero- 
pods whose eggs contain a large and functional food yolk will show 
this similarity. . 
2. After the segmentation of the formative pole has given rise to 
a layer of cells which nearly covers up the single large spherule at 
the nutritive pole the latter begins to divide, and the egg at the same 
time becomes flattened, and finally saucer-shaped, with the concave 
surface formed by a layer of cells which have been produced by the 
division of the single macromere. 
3. The edges of the saucer-shaped embryo approach each other, 
thus converting it into a gastrula, with a primitive digestive cavity 
and an external opening or blastopore. 
4. The edges of this opening continue to approach and finally 
meet and unite, thus surrounding the endoderm on all sides by a 
layer of ectoderm, and leaving its central cavity without an opening. 
5. After the lips of the blastopore have met and united to close 
the opening, its position is indicated for a short time by a groove on 
the outer surface of the body. The two valves of the shell make 
their appearance, entirely separate from each other, at the lateral 
edges of this groove, which thus becomes the shell area. 
