JENNINGS: DEVELOPMENT OF ASPLANCHNA IERRIOKII. 57 
occupied by d?^, the sagittal plane passing through this cell forming a 
plane of symmetry for all. The median plane thus indicated coincides 
with that already defined by the line separating the quadrants A and B 
anteriorly, and the boundary between the cells d and dds. H posteriorly ; 
it passes through the animal pole and the blastopore. 
This movement of the entodermal blastomeres is of course simply a 
continuation of the rotation inaugurated at the passage from the four- to 
the eight-cell stage. The ectodermie cells continually withdraw their 
deeper parts and increase their surface area at division; in this way cells 
are continually forced in at the blastopore. These press upon the anterior 
and ventral aspects of the entoderm cells, forcing them backward, as 
already described. The pressure is greatest in the median region, so 
that the anterior or ventral ends of the cells on the two sides of the 
median plane are forced apart, the axes of the cells become oblique to 
the long axis of the embryo, and the oblique position of spindles shown 
in Figure 83 results. 
Beyond this point it is impossible to trace the development cell by 
cell. In Asplanchna there is a period, intervening between the stage to 
which it is possible to trace the cleavage step by step (about 120 cells) 
and the stage of recognizable differentiation of organs, during which the 
cells divide and become very minute, The cells probably reach the 
number of from 250 to 500, and the process of extension of cells and 
consequent, “rotation ” and invagination of parts of the embryo continues. 
A sagittal section of the embryo at about the time of the beginning of 
differentiation of organs is shown in Figure 84. It thus becomes 
impossible to trace the fato of individual blastomeres, or even, except in 
& most general way, the fate of the different regions of the embryo during 
the later portion of the cleaving. From the small size of the adult roti- 
for, I had hoped that it would prove to be a favorable object for an exact 
study of the cytogenetic history of organs ; in Asplanchna this turns out 
not to be the ease. But, on the other hand, for a study of the factors in 
the early developmental processes it has shown itself well fitted. 
My study of the processes in the early development of Asplanchna there- 
foro closes with the stages shown in Figures 79 and 83 (Plates 9 and 10). 
For critical comparison of my observations with those of Zelinka and 
other workers on the development of the Rotifera, the reader is referred 
to Part Second. 
I shall now proceed to a discussion of the bearing of the foregoing 
observations upon the problems already proposed, as well as upon other 
related subjects. 
