No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 393 



What becomes of this cell in this form would be a matter of 

 considerable interest. 



The Fourth Quartette. — The fourth quartette is produced by 

 a dexiotropic cleavage of the entomeres. All the cells of this 

 quartette are large and full of yolk ; the four cells at the vege- 

 tal pole are small and clearer than the others. They are quite 

 thin and, therefore, less in bulk than their superficial area would 

 indicate. The general appearance of the entoderm cells at this 

 stage is very similar to those of Limax, and markedly different 

 from those of most marine forms, in which there is usually 

 much yolk. 



The number of cells in the egg when the fourth quartette is 

 formed is forty-nine. 



Cells of the first quartette ^g f 8 trochoblasts 



( 8 apical cells 

 Cells of the second quartette 16 



Cells of the third quartette 8 



Cells of the fourth quartette 5 j 



Cells at the vegetal pole 4 



2 mesomeres 



3 entomeres 



One of the most conspicuous features of the Qgg at this stage 

 is a belt of large cells around the equator. It is composed of 

 twelve cells, the four middle pairs of cells of the second quar- 

 tette alternating with the four upper cells of the third. All of 

 these cells are m.ore or less oblong, with their long axes verti- 

 cal. A similar belt of cells may be seen, though less plainly, in 

 Crepidula and Limax. 



The History of the Cross. 



At the time of its first appearance the cross contains eight 

 cells of the first and four cells of the second quartette. Its 

 center is at the apical pole of the ^g^, and its arms are anterior, 

 posterior, right, and left. The tips of the cross lie over the 

 large entomeres of the fourth quartette, the cells of the second 

 quartette lying between. The median plane of the ^gg would 

 cut through the middle of the anterior and posterior arms, and 

 the entomere i\b, and pass between the two mesoblasts. Pre- 

 vious to the formation of the fourth quartette, this plane would 



