No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 



415 



figures show very clearly the history of the process. The deeply 

 staining bodies occur in all the cells at an early period, even in 

 the sixteen-cell stage, and increase in number as the develop- 

 ment proceeds. The cells of the ectoderm take in the albumin 

 more rapidly than in Planorbis ; but, finally, this function is 

 transferred entirely to the cells of the entoderm. The process 

 of digestion of albumin, by which the &gg is nourished and 

 enabled to grow, is carried on at first with equal facility by all 

 the cells of the ^^g. This process soon predominates in the 

 entomeres, although, for a time, it is carried on more rapidly 

 by all the cells. Finally, the ingestion of albumin is relegated 

 to the cells of the enteron, which have become specialized for 

 digestive purposes, and the other cells of the Qgg no longer 

 share this function. 



TJie Rudiments of the Cerebral Ganglia and Eyes. 



The two portions of the cross, which are separated by the 

 median apical plate, form the regions which give rise to the 

 cerebral ganglia and eyes. These two rudiments, in eggs 

 stained with silver nitrate, appear as dark masses surrounded 

 on every side by large and very transparent cells. The tip 

 cells of the lateral arms, and the cell lying immediately above 

 them, do not enter into the formation of these masses, but 

 increase in size and go into the head vesicle. With the excep- 

 tion of these two cells in each arm, all the cells in the lateral 

 arms of the cross, the cells of the anterior arm, except the tip 

 and basal cell, and the central region of the cross, except the 

 four apicals and the two cells lying in front of them, enter into 

 the formation of these two rudiments. The composition of 

 these two patches of cells when they are definitely marked off 

 may be seen in the following table : 



Right Rudiment. 



j^l.2.2.1.I 



I3I.2.2.I.2 



1^1.2.1.2.1.1 



j^l.2.I.2.I.2 



1^1.1.2.1 



1^1.1.2.2 



