JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 49 



those in quadrant D. Figure 61 (Plate 7) shows the anterior surface of 

 the egg at the end of the sixth cleavage, and a diagram of a single 

 quadrant at this stage was given on page 41. Comparing either the 

 figure or the diagram with the scheme (Diagram III.) given on page 46 

 for the quadrant D at the same stage, the aiTangement is seen to be the 

 same except in two respects. (1) The cells belonging to the same lat- 

 eral series are equal in A, B, and C, unequal in D. (2) Four ventral 

 ectodermal cells additional are present in each of the quadrants A, B, 

 and C; these are represented in D by the cells which have passed 

 inward to form the entoderm. 



As shown by the spindles in Figure 61, the first cells to divide are 

 ^7.i3_g7.i3 jj^jj(j ^7.i4_g7.i4 rpj^g clcavago is meridional and equal. The 

 resulting cells are a^-^-c^-"^, a^-^^-c^-'^, o«-2^-c«-% and a8-2«-c»-2«. They 

 are shown in Figures 72 and 75 (Plate 9). (Compare Diagram Y.) 



Next follows the division of the six cells, dJ-^-c'-^ and dJ-^-c^-^, which 

 together form part of a transverse girdle, surrounding the eg<^. The 

 cleavage is meridional and equal. The resulting 12 cells, a^^-c^-", o*-^-- 

 ^.12^ ^8.i5_g8.i5^ j^^(j ^8.i6_g8.w forming as before a ti'ansverse girdle, are 

 shown in Plate 8, Figs. 69 and 70, and Plate 9, Fig. 71. 



Next ensues the cleavage of the transverse row containing the six 

 cells, dJ-^-c^-^ and a^-'^-c'-'' . These cells, as shown in Figure 61 (Plate 7), 

 are much flattened dorso-ventrally, and are of exactly the same form as 

 the cells last discussed, which lie immediately dorsad of them. Moreover, 

 each cell in this row corresponds in origin to a cell of the row last 

 described, the two rows having been derived from the equatorial division 

 of a pi'eviously existing transverse row, as shown in Figure 55. If 

 mechanical conditions are decisive in determining the direction of the 

 cleavage, these two rows should cleave in the same manner, i. e. both 

 meridionally. Nevertheless, as shown in Figui-e 69 (Plate 8), while the 

 dorsal of the two rows divides meridionally, the cells of this ventral row 

 all cleave equatorially. The axis of the cell in which the spindle lies is 

 about half as long as the axis which is at ri^ht angles to it. The cells 

 elongate in the direction of the spindles, and a very unequal division 

 ensues. The ventral products are minute, while the dorsal ones are 

 nearly equal in size to the mother cells. Figure 75 (Plate 9) shows 

 the anterior surfiice of the Qg^ after this cleavage. The twelve cells 

 produced are a^-^-c^-^, a*-'*'-c*-^°, a^-^^-c^-^", and a*"-c^-". 



The division of the band of twelve small cells composed of cl'-^-c'-^, 

 „7.io_^7.io^ a7.;i_c7.n j^,^^ ^,7_i2_g7.i2_ ^nd shown in Figure 61 (Plate 7), 

 follows somewhat later. The cleavage is equatorial and the spindles lie 



VOL. XXX. — NO. 1. 4 



