378 RICHARD EVANS. 



usual rule, the granules are evenly distributed throughout the 

 whole nucleus^ and are united to one another and to the 

 nuclear membrane by means of threads. These differences, 

 however, are only differences of degree, and not of quality, for 

 there are always a great number of intermediate stages. 



To sum up, the flagellated cell may be described as elongated, 

 and constricted at the middle. The nucleus, situated at the 

 base of the cell, has a thick membrane enclosing a network 

 of threads, at the nodes of which from one to five chromatin 

 granules are situated. 



(b) The Inner Mass. — The inner mass may consist of as 

 many as three kinds of cells, of which two at least are always 

 present. Without any further general remarks I shall pro- 

 ceed to describe these three kinds of cells, leaving certain 

 questions to be dealt with in the appendices, e. g. the origin of 

 the '^ cell groups," the enclosures found chiefly in the cells 

 with vesicular nuclei, and the development of the spicules and 

 changes which take place in the nucleus of the scleroblast. 



(a) The Cells with Granular Nuclei. — These cells are 

 found to occur in two positions, at least, out of the three which 

 they may occupy. They are always found as a more or less 

 complete layer immediately under the flagellated cells, and as 

 the larva grows older the completeness of the layer becomes 

 more evident. Secondly, they are always present in the imme- 

 diate vicinity of the larval cavity, especially the anterior moiety 

 of that cavity ; and here, again, the layer becomes more com- 

 plete with age. And thirdly, they may occur in the interior 

 of the solid part of the inner mass, where, however, they do 

 not appear to be present at first, save in exceptional cases, and 

 even then only in very small numbers. 



In the first and second of these positions the cells with 

 granular nuclei tend to become flattened, and consequently in 

 a radial section present an oval form ; while in the third they 

 present an irregular shape, pushing their processes between the 

 other elements of the inner mass. As the larva becomes older 

 they increase in number at the expense of the cells with vesicular 

 nuclei as well as by their own division. The proportion which 



