No. 3. ] DEVELOPMENT OF MARINE SPONGES. 345 
In the transformation of the simple embryo, Fig. 111, into 
the ciliated larva, the outer cells become the ectoderm, the 
inner cells forming an undifferentiated mass, the parenchyma 
or mes-entoderm. When the embryo escapes from the parent 
and begins its free swimming life, it is in the condition shown 
in Fig. 112 (longitudinal section). The ciliated larva is of an 
oval shape, one end being considerably broader than the other, 
and of a uniform brown color. It is ciliated all over, there 
being no differentiation of an unciliated, unpigmented pole as 
in the gemmule larva of Tedania and Esperella. I did not 
follow the further development of the swimming larva, but it is 
quite possible that an unciliated pole may later make its ap- 
pearance, as in the Desmaczdon described by Barrois. 
The ectoderm of the larva is uniformly composed of very 
long slender cells, the peripheral ends of which contain the 
nuclei and being free from yolk form a zone clearly marked off 
from the rest of the larva, Fig. 112. In this zone the outlines 
of the ectoderm cells are plain enough. The ectoderm cells, 
however, extend a long distance internally from this zone, and 
their inner portions containing fine yolk, similar to that with 
which the mes-entoderm cells are filled, the cell outlines are 
here not very distinct. The parenchyma at first sight appears 
to be a uniformly granular matrix containing nuclei. But in 
very thin sections its cellular nature can be made out. It is 
composed of irregularly polygmal cells, which are so closely 
appressed and so full of fine yolk granules, that the cell boun- 
daries are obscured. 
In sectioning the parent sponge for embryos, I came across 
the curious case of attachment illustrated in Fig. 143. A cili- 
ated larva of an irregular shape, and containing two or three 
flagellated chambers, is present in one of the larger canals, 
and appears to have attached to the wall of the canal instead 
of passing out of the body of the mother. There is, as can be 
seen in the figure, a perfect continuity between the mesoderm 
of the parent and the parenchyma of the larva. The columnar 
ectoderm on the other hand does not seem to be continuous 
with the epithelioid lining of the canal, but rather to pass into 
the mesoderm of the adult through a break in the canal wall. 
