JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 29 



This cleavage, as described above, differs in some respects from that 

 of Callidina as described by Zehnka ('91). A discussion of the differ- 

 ences will be found in Part Second. 



The cleavage of Asplanchna priodonta takes place to this stage in 

 exactly the same manner as that of Asplanchna Herrickii. Figure 29 

 (Plate 4) shows the egg of Asplanchna priodonta in the 10-cell stage. 



Fifth Cleavage. 



As a basis for an account of the following cleavage, it will be well to 

 summarize the divisions which have already taken place, and to take a 

 careful survey of the structure of the egg at the end of the fourth 

 cleavage. 



The first and second cleavages pass through both animal and vegeta- 

 tive poles and are therefore meridional. The third cleavage is at right 

 angles to the dorso-ventral axis and is therefore equatorial. The fourth 

 cleavage is pai-allel to the third, thus likewise equatorial. 



As a result of these cleavages, the egg now consists of sixteen cells, 

 arranged in four dorso-ventral rows or quadrants, each quadi'aut con- 

 sisting of four cells derived from one of the four blastomeres of the 

 four-cell stage (Plates 4 and 5, Figs. 30-36). Passing from the ven- 

 tral side dorsad, we may also distinguish four layers of cells, each layer 

 containing one cell of each of the four quadrants. The layers may for 

 convenience be numbered ; I will call the ventral the jirst layer, the 

 others following in order to the fourth, which is at the animal pole. As 

 a result of the shifting during cleavage, the animal pole has now come to 

 be situated almost exactly at the micromere end of the egg : the oppo- 

 site end is occupied by the large cell d^-^ (Figs. 30 and 33, anterior 

 and posterior views respectively). The dorso-ventral axis therefore 

 now coincides with the long axis of the egg. 



Of the four quadrants, three. A, B, and C, are alike in the size and 

 arrangement of the cells of which they are composed. (See Fig. 30, 

 anterior view.) The four cells composing any given one of these quad- 

 rants differ in size. The cell of the fourth layer (next to the animal 

 pole) is much the largest, while that of the third layer is much the 

 smallest. The cells of the first and second layers are nearly equal in 

 size ; that of the first layer covers more of the surface of the Qgg (Fig. 

 30), but that of the second layer is deepest (Fig. 32). The cells of the 

 first, second, and third layers are much compressed dorso-ventrally, so 

 that the lateral dimensions of the cells are at least twice as great as the 

 dorso-ventral dimensions. In the third layer especially, the cells are 



