JENNINGS: DEVELOPMENT OF ASPLANCHNA HERKICKII. 



53 



entirely closed, as shown in Figure 73.) The blastopore notch lies, not 

 directly at the macromere end of the egg, but at some distance on the 

 posterior side of it. 



As the cells of the anterior lip of the blastopore are turned inward, 

 some of them begin to divide. Spindles in these cells are shown in 

 Plate 9, Figs. 75 and 77. The form and position of the cells have 

 changed so much at this time that it is impossible to detei'mine with 

 certainty whether the cleavage should be considered equatorial or merid- 

 ional. In the cases figured the spindles are nearly or quite transverse, 

 so that in some of the cells the division is meridional. 



Diagram YI. 



Diagram of quadrant D at a time when 

 most of the cells have passed into the 

 ninth generation. All cells in the ninth 

 generation are bounded by continuous 

 lines, and are designated by the second 

 exponent belonging to the cell, the first 

 exponent being in each case 9. In the 

 other cells both exponents are given. 

 The arrows connect cells of common 

 origin, and show the direction of the 

 spindle at the last division. 



R signifies right; L, left; D, dorsal; 

 V, ventral. 



The cleavage of the ectoderm has now been traced to the eighth gen- 

 eration in all parts of the egg, and in the greater part of the quadrant 

 D to the ninth generation. 



With this ends the account of the cleavage of the ectodermal cells. 

 The small size and the great displacements of the blastomeres, espe- 

 cially in the regions of the blastopore and the animal pole, render it 

 impossible to determine with certainty their identity in later stages, 

 and the real direction of cleavage is masked by crowding and deforma- 

 tion of the cells. It would perhaps be of little intei-est in connection 

 with the laws of cleavage to carry the study further, as it is scarcely to 

 be presumed that the later divisions would exhibit any phenomena dif- 



