SUDLER: DEVELOPMENT OF PENILIA. 
115' 
tiation. All of the planes of division cease to be distinguishable' 
as such from this time. (Fig. 12.) 
The seventh stage probably has about one hundred and twenty- 
eight cells and shows the beginning of gastrulation. (Fig. 13.) 
Gastrulation. 
This process always takes place in a very definite part of the 
egg with reference to its position in the mother. 
No matter on which side of the intestine of the adult the egg 
lies, gastrulation always occurs at its outer posterior corner. An 
exception to this rule was not observed in the scores of specimens 
examined at this stage. A median horizontal longitudinal section 
(Fig. 12) at this time shows projecting into the segmentation 
cavity from the outer and posterior angle of the outer wall of the 
egg an indefinite amount of protoplasm containing one or two 
nuclei. At the particular point to which this is attached to the 
outer wall one or rarely two cells have sunk below the surface 
and are in the process of migrating into the interior. (Fig. 13.) 
The nuclei and protoplasm shown in the interior have reached 
this position by a similar process. In all probability regularity in 
the planes of division has disappeared even before this time, but 
it certainly has now. The cells, however, continue to divide at 
nearly the same time. An older embryo (Fig. 14) shows that both 
the amount of protoplasm and number of nuclei in it in the seg¬ 
mentation cavity have increased and a larger number of cells are 
concerned in the inwanderino; and formation of the mouth of the 
gastrula. 
In Moina, gastrulation begins later than in Penilia, and apparently 
the process is more rapid since more cells are concerned from the 
beginning. In an older Penilia larva this indefinite protoplasmic 
mass has become quite a definite rod running through the segmenta¬ 
tion cavity and contains numerous nuclei. The gastrula mouth is 
much deeper and enlarged. No differentiation except that noted 
can be seen. Cell walls are now made out with great difficulty, if 
at all, although the individual cells stand out as such very plainly. 
Boundaries between them are marked off by lighter staining areas 
of protoplasm. (Fig. 15.) At a corresponding or even earlier 
stage, Moina, according to Grobben, already shows the beginning 
