384 BlOHAED l^VANS. 



Special Features of Type B. 



Ill this type the iuuer mass consists of three kinds of cells, 

 namely, cells with granular nuclei (a), with vesicular nuclei 

 (j3), and cell groups (y). 



The cells with granular nuclei are now sufficiently developed 

 to form fairly complete layers in the two positions in which 

 they were recognised in type A, and have also appeared in the 

 interior of the solid posterior end of the inner mass. They 

 have also become flattened save in the interior, and their nuclei 

 are in some cases subspherical in form. 



The cells with vesicular nuclei are far less numerous than 

 in type A. They have become slightly smaller in size, and 

 less definite and regular in outline. The nucleus also is 

 smaller, and the central corpuscle is evidently diminishing 

 in size and breaking up. These facts point to the gradual 

 change of some of the cells with vesicular nuclei to such as 

 have granular nuclei. 



The cell groups which occur in this type are a new and 

 most important feature which did not exist in type A. 

 The cytoplasmic bodies of the individual cells which make up 

 the groups are as yet incompletely divided from one another, 

 so that the nuclei present the appearance of lying at the 

 peripiiery of a mass of cytoplasm. It is only when a group is 

 looked at in surface view that anything resembling a dividing 

 line between the nuclei can be seen, a fact which indicates 

 that the division of the multinucleated cytoplasmic mass 

 proceeds slowly from the surface towards the centre (figs. 

 9 a and b, (j. c). The amount of cytoplasm corresponding to 

 each nucleus is not more than that contained in a flagellated 

 ecll. The nuclei in a radial section of a group present the 

 same onion-shaped form as those of the flagellated cells. The 

 nuclear membrane is thick, and the granules are small and few. 

 In short, these cells, while diff'eriug completely, on the one 

 hand, from the cells with granular nuclei, both as regards the 

 size of the cell and the characters of the nucleus, are almost 

 identical, on the other hand, in both these points with the 



