JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 39 
ure 57. In this figure, d*5 has already divided into d"? and die, the 
nuclei of which are still connected by interzonal filaments. In dere 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 des and ; in these it is unequal, The unequal 
cleavage of cs is shown in Figure 58, and the same figure shows the 
unequal products (Gu and de) resulting from the division of d 
3y the division of the third layer a band of sixteen cells is produced, 
extending completely around the embryo. The cells composing the 
band are a? =d", 7.40 _ Geo, a. 1 dH, and a - a", 
Fourth Layer, containing the four cells a°" — d°. 
In these cells the cleavage is meridional, as in the third layer, and in 
every case equal. The spindles in 097 and c, are shown in Figure 47 
(Plate 6). 
The eight cells resulting from this cleavage are a^? e and 
G4 qa 
Fifth Layer, containing the four cells a*5 de, situated at the animal 
pole of the egg. 
The four small cells at the animal pole of the egg divide equatorially. 
The spindle of des is shown in Figure 59 (Plate 7) ; and of de' in Fig- 
ure 60. The cleavage products are very unequal; the dorsal cells so 
formed are very minute, so that the distinetion 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 tho cleavage 
at the animal pole completed except in the cell ds. A group of three 
small vesicles, representing the dorsal cleavage products of the cells 
50.8 — Ges, lie at the animal pole, surrounded by the four larger cells, — 
one of which is the undivided cell des, while the others are the ventral 
cleavage products of 095 s. 
The eight cells thus produced are a= d:5 and de — dt 
This, the sixth, cleavage may be tabulated as follows: — 
Direction of Nature of 
Layer. Cella, savage, Cleavage, Product, 
First, or (4, 6, c,d) ®t and 92 Equatorial Equal (a-d) ™ and 7, (a-c) 
Ventral (except d) (exe. di) 73 and 7*4 [and de 2J. 
Second (a, b,c, d) 9*9 and &4 Equatorial Equal (a-d) "5 and 79; 
(exc. d9:8) (a-d) 77 and 78, 
Third (a,b,c,d) *5 and 99 Meridional Equal (a- d) "9 and 710; 
(exe. 066, dë) (a — d) TH and 72. 
Fourth ele re at Meridional Equal (a-d) 133 and 714, 
Fifth (a, O, c, d) 98 Equatorial Unequal (a -d) 15 and 7:19, 
