JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 4b 
itself diminished in size. The interior of the gastrula is occupied by 
the large cell d" and the two minute vesicles d*? and d^? (Fig. 64). 
The interior cells are surrounded by a single layer of outer cells, except 
at the animal pole of the egg, where the small dorsal cells do not reach 
to the cells within the gastrula, but lie on the surface, making here a 
two-layered region. "This region remains two-layered as long as it is 
possible to trace the history of the animal pole of the egg. 
The three inner cells di, d., and d"?, with the products of the 
former, may henceforth be called the entoderm, the outer layer the 
ectoderm. 
Seventh and Later. Cleavages. 
I have followed the cleavage through another generation, and, for 
parts of the egg, much farther. It becomes impracticable, however, to 
describe the eleavage according to the layers or series in which it takes 
place, as has been done up to this stage, owing to the complicated suc- 
cession of the divisions in the different cells, and to the great changes in 
position taking place while the cleavages are in progress. I shall there- 
fore now describe the processes in the general order in which they 
occur, and in so doing I shall consider separately (1) the ectoderm, and 
(2) the entoderm. 
Tue ECTODERM. 
In discussing the changes taking place in the ectoderm, it will be well 
to distinguish for convenience of description two regions : (1) the (left) 
posterior part of the ectoderm, derived from the quadrant D; (2) the 
anterior and right lateral portions of the ectoderm, derived from the quad- 
rants A, B, and C. While the phenomena occurring in all of these regions 
are reducible to the same ‚general scheme, so far as the method of 
cleavage is concerned, the irregularity in the size of the blastomeres 
forming quadrant D, their earlier cleavage as compared with the other 
quadrants, and the fact that some of the cells have passed inward to 
form the entoderm, give this region a peculiar and somewhat irregular 
charaeter, which makes it convenient to discuss it separately. 
(1) The Quadrant D. — The entoderm cells belong genetically to this 
quadrant, but they will be considered later. 
In order to understand the conditions in quadrant D, and to see their 
relations with the arrangement in the other quadrants, it will be well to 
emphasize certain features of the last two cleavages. 
At the fifth cleavage, as previously deseribed, the three ventral cells 
of quadrant D divided by meridional planes into unequal portions, the 
left derivative being in every case larger (Plato 4, Fig. 33, and Plate 5, 
