No. 3-] DEVELOPMENT OF MARINE SPONGES. 345 



In the transformation of the simple embryo, Fig. lu, 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 Fi- 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 

 bein- no differentiation of an unciliated, unp.gmented 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 Dcsniacidon described by Barrois. 



The ectoderm of the larva is uniformly composed of very 

 Ion- 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 outlmes 

 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 outlmes are 

 here not very distinct. The parenchyma at first sight appears 

 to be a uniformly granular matri-K containing nuclei. But m 

 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 m Fig. 113. A cili- 

 ated larva of an irregular shape, and containing two or three 

 fla-ellated 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. 



