STUDIES ON THE COMPARATIVE ANATOMY OE SPONGES. 351 
cells are embedded. The outer layer, or ectoderm, consists of 
a single layer of large, sac-shaped or somewhat flask-shaped 
cells (figs. 19, 20, 21, 22) measuring about 0’024 mm. in 
length. The narrower portion, or neck, of the cell is on the 
outside of the embryo, and the swollen portion projects in- 
wards into the gelatinous intercellular substance, and from its 
inner extremity frequently sends out a few very short, slender 
pseudopodial processes (figs. 19, 20, 21, 22). These processes 
may possibly serve to transmit nutriment to the cells in the 
interior, or they may simply indicate a tendency in the ecto- 
dermal cells to become amoeboid. The body of the cell is 
coarsely granular, but less so in the neck than in the swollen 
portion. The greater part of the neck is, however, occupied 
by a large, spherical nucleus, which appears to consist of a 
hollow vesicle containing a few deeply-staining granules. I 
have observed no nucleolus. The nuclei are all arranged at 
just about the same level, so that the nuclei of adjacent cells 
form a continuous row, which is a very conspicuous feature in 
sections of the embryo. 
Frequently the outer end of the neck of the ectodermal cells 
may be seen to be drawn out into a short, slender, protoplasmic 
process, which extends to the outer surface of one of the large 
investing epithelial cells (fig. 21), and attaches itself to the 
latter. Thus the ectodermal cells of the embryo often appear 
to be suspended from the outer surfaces of the investing epi- 
thelial cells by short protoplasmic processes, as shown some- 
what diagrammatically in fig. 21. Judging from the number 
seen in a single transverse section, it would appear that each 
of the large epithelial cells may have a hundred or more sac- 
shaped ectodermal embryonic cells hanging from its outer 
surface. 
The unusual length of time during which the embryo 
remains within the mother Sponge, and the great size to which 
it attains, necessitate some special arrangement whereby it 
can be nourished. The peculiar relation of the ectodermal 
cells of the embryo to the investing epithelium, and the very 
unusual character of the latter, cause me to believe that the 
