330 WILSON. [Verde 
nucleate masses, as in the previous stage. Most of the mes- 
entoderm cells have, like the ectoderm cells, plump bodies full 
of fine yolk granules, but there are some with smaller slender 
bodies, in which there is but little yolk and which begin to 
assume the appearance of the spindle-shaped cells, so abundant 
in the older embryo (compare Pl. XXI, Fig. 81). The pieces 
of spicules found here and there in embryos of this stage were 
already present in the gemmule before it began to break up. 
It will be noticed in this stage, Fig. 75, that the ectoderm 
cells are, in many instances at any rate, connected with the 
mes-entoderm cells by fine terminal processes. This connection 
probably continues to exist in the later stages, but I did not 
satisfactorily demonstrate it. 
The ectoderm cells which already form a distinct layer in 
Fig. 75 divide in planes vertical to the surface, and become 
long slender columnar cells. These slender columnar cells 
form for a time a uniform investment for the whole embryo, 
Pl. XX, Fig. 76, though later they flatten out over one pole. 
In Pl. XXI, Fig. 76, is shown a small part of Fig. 76, more 
highly magnified. The rounded multilobed masses of the 
earlier stage are no longer found in the mes-entoderm, which 
now consists only of separate cells. 
While the embryo is still in the body of the mother and 
surrounded by its capsule, the ectoderm cells. over one of the 
poles flatten out, while elsewhere they develop cilia and become 
deeply pigmented. In. Pl. XX, Fig.77, is shown a> section 
through this pole of the embryo at a stage just before the 
flattening has begun. The ectoderm cells over the general 
surface have flagella, and nuclei near their lower ends, the 
nuclei forming a zone several layers thick. The ectoderm 
cells at the pole, however (ec. uu-p. p.), are not quite as slender 
and have no flagella. Their connection with the cells of the 
mes-entoderm is still obvious. These cells gradually flatten 
until, by the time the embryo leaves the body of the mother, 
they have assumed the character shown in Pl. XXI, Fig. 78 
(ec. un-p. p.). The mes-entoderm in the stage shown in Fig. 77 
is much as in the earlier stage, with the exception that a 
number of spicules are now scattered through it, all of them 
