JENNINGS : DEVELOPMENT OF ASPLANCHNA HEREICKII. 45 



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. G4). 



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 d'-\ 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 cleavage 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. 



The 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 

 character, 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 described, the three ventral cells 

 of quadrant D divided by meridional planes into unequal portions, the 

 left derivative being in every case larger (Plate 4, Fig. 33, and Plate 5, 



