No. 3-] DEVELOPMENT OF MARIXE 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 PI. 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 



