JENNINGS : DEVELOPMENT OF ASPLANCHNA HERPJCKII. 39 



ure 57. In this figure, d^'^ has already divided into d'''^ and c?'^*^", the 

 nuclei of which are still connected by interzonal filaments. In c?®*® the 

 spindle is still present. The completed division in the anterior part of 

 the egg is shown in Figure 61. The cleavage is equal in all the cells of 

 this layer except d^-^ and c^'® ; in these it is unequal. The unequal 

 cleavage of c^'^ is shown in Figure 58, and the same figure shows the 

 unequal pi'odiicts (d''^^ and d"^-^-) resulting from the division of d''-^. 



By the division of the third layer a band of sixteen cells is produced, 

 extending completely around the embryo. The cells composing the 

 band are o^-^-rf^-^, a^-^^-rf^-^ a^-"-rfT.n ^^^^ ^i.i-2_^^.u 



Fourth Layer, containing the four cells a^-'^-rf®*". 



In these cells the cleavage is meridional, as in the third layer, and in 

 every case equal. The spindles in S®*' and c^'' are shown in Figure 47 

 (Plate 6). 



The eight cells resulting from this cleavage are a'-'^^ -d''-^^ and 



„7.14_^7.14 



Fifth Layer, containing the four cells a^-^-d^-^, situated at the animal 

 pole of the egg. 



The four small cells at the animal pole of the egg divide equatorially. 

 The spindle of d^-^ is shown in Figure 59 (Plate 7) ; and of a^-^ in Fig- 

 ure 60. The cleavage products are very unequal ; the dorsal cells so 

 formed are very minute, so that the distinction between cell body and 

 nucleus cannot be observed, and the cells cannot be distinguished from 

 the polar cell lying in the same region. Figure 60 shows the cleavage 

 at the animal pole completed except in the cell a^-^. A group of three 

 small vesicles, representing the dorsal cleavage products of the cells 

 j6.8_^6.8^ lie at the animal pole, surrounded by the four larger cells, — 

 one of which is the undivided cell a®-^, while the others are the ventral 

 cleavage products of h^-^-d^-^. 



The eight cells thus produced are a''^^ -d'-'^^ and a''-'^^ -d"^'^^. 



This, the sixth, cleavage may be tabulated as follows : — 



Product. 



(a -(I) "1 and '■'^, (a-c) 

 "■3 and '■* [and (/6-2]. 

 (a-d) "•5 and 7-6; 



(a-rf)7-'^and'-8. 

 {a-d) 7-9andT-iO; 

 (a-t/)T-iiand'-i2. 

 (a-(7)7i3and"-". 

 [a-d] "-15 and '-IS. 



