No. 3.] DEVELOPMENT OF MARINE SPONGES. 297 
There then follows a clear area (c.a.) in which no large granules 
are found. A coarsely granular region (g.a.) comes next. The 
nucleus is always at the lower end of the cell, which terminates 
in a delicate process. The ectoderm cells over the posterior 
pole are of the sort shown in Pl. XVII, Fig. 33, which represents 
a maceration preparation of this region. The bodies of the 
cells extend down in an irregular fashion into the mass of 
parenchyma, and they take a deep stain with haematoxylin. 
The transition from them to the ciliated and columnar ecto- 
derm is an abrupt one, as may be seen in the section, Fig. 29. 
The parenchyma of the swimming larva is considerably more 
differentiated than in Fig. 25. The cells in the posterior part 
of the body, Fig. 29, are closely packed and polygonal. They 
stain feebly and their cell outlines are indistinct. In front of 
these cells and about in the middle of the body, is a region 
containing a large number of cells with plump, finely granular 
bodies, taking the stain well. These cells are of special im- 
portance in building up the internal tissues of the sponge and 
may be spoken of as formative cells. The formative cells are 
rounded or amoeboid in shape, with slender processes which 
connect the cells together. In Pl. XVI, Fig. 32, a group of 
such cells, as seen in a maceration preparation, is shown. The 
anterior part of the larva is largely occupied by fusiform cells 
with small bodies, taking the stain very feebly, and terminating 
at each end in a slender process. Scattered here and there 
amongst the fusiform cells are a few well-staining granular 
cells. It is probable from the structure of the earlier larva as 
well as the later, that all the cells in this stage are connected 
together by processes. But in macerations this was only 
clearly brought out in the case of the formative cells. The 
direction of the fusiform cells round the periphery (Fig. 29) 
probably indicates a connection between them and the slender 
terminal processes of the ectoderm cells, Fig. 31. 
In the swimming larva there are three kinds of spicules 
present. Imbedded in the mass of pale polygonal cells of the 
posterior end are a number of straight spicules (oxytylotes) 
arranged in a loose bundle with their sharp ends pointing 
towards the posterior pole. These spicules very often are 
