Brooks.] 326 [November 19, 
becomes the mouth, some that it becomes the anus, some that it 
becomes neither, and some that it becomes both. In the present 
state of our knowledge it is impossible to say how far this lack of” 
agreement is due to an actual difference in the history of the digestive 
tract in different molluscs, and how far to imperfect or erroneous 
observations. ‘That it is in part the result of error is shown by the 
fact that the account of the various observers who have studied the 
development of thé same mollusc, are often as contradictory and irree- 
oncilable as the papers on different classes or orders. 
I have been engaged lately in studying the embryology of a num- 
ber of molluscs belonging to different classes and orders, and in a few 
of them I have traced all the stages in the formation of the digestive 
tract, and have obtained results which seem to me to be perfectly 
satisfactory and conclusive as far as these forms are concerned. An 
illustrated paper which is now in print, upon the development of 
the Oyster, and another which is also in press, on the development 
of the fresh-water Pulmonates, will give a detailed account of my 
observations upon the history of the digestive tract in these forms, 
and the present paper contains only an outline statement of the 
points in regard to which I feel certain. 
A comparison of the papers upon the development of the Pulmon- 
ata which have been published within the last few years by Ray Lan- 
kester, Rabl, Fol, Ganin and von Ihering, will furnish an ample 
reason for the revision of the embryology of this group, for there is 
hardly a point in the early history upon which these observers agree. 
The following is a list of those leading points in the development of 
Pulmonates of which I feel confident : 
1. The polar globules make their appearance at the formative 
pole of the egg, and mark the plane of first cleavage, and the 
principal axis of the egg. 
2. After the egg has divided into four equal spherules, the proto- 
plasm of each spherule undergoes a process of segregation; that which 
occupies the formative end, and which is destined to give rise to the 
ectoderm, becoming quite transparent, while that which occupies 
the nutritive pole is opaque and granular. 
3. The formative ends of the four primary spherules separate as 
four micromeres, while their nutritive ends become converted into 
four much larger, opaque macromeres. 
4, By the division of the micromeres and by the separation of 
others from the formative ends of the macromeres a layer of cells, the 
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