114 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
Sections of eggs throughout their development show through the 
protoplasm a sharply defined and delicate reticulum with coarse 
meshes. In the young stages and with high magnification, small 
enlargements can be distinguished where the threads meet to form 
the meshes. 
The third plane of division is longitudinal but perpendicular to 
the second, forming an eight-celled embryo in which the cells of 
the anterior end are still slightly larger than those of the posterior 
end. (Fig. 9.) 
In the fourth stage of cleavage the eight cells have divided 
transversely making sixteen cells. (Fig. 10.) Two planes of 
division were necessary to cause this. The eight anterior cells can 
still be seen to be a very little larger than the eight posterior ones. 
At this stage Grobben has already distinguished a cell as the 
mother-cell of the genital organs, another as the primitive ento¬ 
derm cell and still others surrounding the “ genitalzelle ” which 
are thought to give rise to the mesoderm. Thus at this early stage 
these few cells foreshadow all of the germ layers as well as the sexual 
cells of the adult. These cells are distinguished 1st by their peculiar 
staining properties and 2d by differences in their action; i. e. 
time of division in reference to the other cells of the egg. Samassa 
in his studies on the same animal finds the so-called “ Grobben’sche 
zelle” (“ genitalzelle ” of Grobben) and the entoderm present but 
not distinguishable to the same extent as Grobben claims. He is 
unable to distinguish the “ Grobben’sche zelle ” after the original 
one has divided into eight. He fails to find that it develops into 
any particular organ of the adult. In Penilia I can find no cell 
marked from its fellows by any peculiarity at this stage. The cells 
vary somewhat as to size but apparently not in such a definite 
manner as on that account to allow any particular one to be chosen 
and its history followed. 
In the fifth or thirty-two-celled stage regularity of cleavage 
planes is hard to observe and probably does not occur with exact¬ 
ness. From the arrangement of the cells, however, the planes 
must be longitudinal again in their general direction. It is no 
longer possible to distinguish any difference in the size of the cells 
lying at the anterior end from those at the posterior. A cross- 
section of this stage shows segmentation to be complete with a 
large segmentation cavity containing no yolk. (Fig. 11.) 
The sixth or sixty-four-celled stage still shows no sign of differen- 
