116 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
of the nervous system. Samassa states that the embryo is more 
advanced before such a differentiation can be distinguished. 
Size of Embryos. 
So far in Penilia the embryos have grown but little, the elements 
composing them simply becoming smaller with each succeeding 
stage ; but from this time on, increase in size of the whole is marked 
and rapid. In this respect it agrees with the development of Moina. 
The embryos and eggs from the same mother are all of the same 
size and stage, as far as can be detected, throughout their develop¬ 
ment. Embryos of like stages but from different mothers vary 
somewhat in size; a younger stage is larger than a more advanced 
one from another animal. Grobben found the same difference in 
size present in the embryos of Moina and thought the cause was 
traceable to variation in the size of the eggs when first laid by the 
different mothers. This furnishes a partial explanation for the 
phenomena seen in Penilia as the early segmentation stages of this 
animal show the same differences in size, but never to the same 
extent that the older embryos do. The amount and rapidity with 
which the nutritive blood-plasma is supplied to the embryos also 
vary in all probability, and this may be the principal cause in the 
greater variation in size seen in the older embryos. 
The Germ Layers. 
The protoplasmic rod occupying the segmentation cavity is com¬ 
posed almost entirely of the entoderm. The mesoderm originates 
on either side of the mid-ventral line at the angles of the gastrula, 
which at these particular points cannot be separated definitely into 
entoderm and mesoderm. Certain cells, instead of growing up into 
the center of the segmentation cavity, project out into these angles 
and toward the ectoderm. As they multiply they increase in length 
and width, but are only one-layered in thickness. They leave the 
entoderm completely and cling to the inside of the ectoderm. 
(Fig. 16.) These and their descendants form the mesoderm of the 
adult animal. Samassa finds on the ventral side of Moina a mass of 
tissue somewhat semilunar in cross-section. The central-lying cells 
form themselves into a solid cylindrical row. These are the ento- 
