368 RICHARD EVANS. 



layer under the flagellated cells as well as lining the larval 

 cavity, and it may be in other parts of the inner mass. They 

 are irregular in shape, but may be very flattened when they 

 border a cavity. 



The nucleus is spherical or subspherical, with small irregu- 

 larly shaped chromatin granules, more or less equal in size, 

 situated at the nodes of an even reticular framework. 



The cytoplasm may contain a few yolk bodies (figs, la, 5, 

 and 6; c. g. n.). 



(j3) Cells with vesicular nuclei aggregated chiefly towards 

 the centre of the solid posterior region of the larva. They are 

 massive cells, spherical or oval in form, especially in the 

 younger larvse, but sometimes in later stages quite irregular in 

 shape. The cells with vesicular nuclei represent a class of un- 

 modified cells derived from the blastomeres from which the 

 other cell elements arise, and which therefore diminish in 

 number during the progress of the development. Their nucleus 

 possesses a large central corpuscle suspended in a delicate 

 nuclear reticulum which always contains granules, varying 

 in number, size, and distribution. The cytoplasm contains 

 from one to four '' nutritive vacuoles " and several or numerous 

 '' yolk bodies " (figs, 1, 2, 5 a and b, 6 a, and 7 ; c.v. n.). 



(7) Small cells arranged in groups, which may be termed 

 briefly ''cell groups.'' They are always situated in the interior 

 of the solid posterior part of the inner mass. In many cases 

 the component cells of a group are not completely separated 

 from one another, but present the appearance of nuclei arranged 

 near the surface of an incompletely divided mass of cytoplasm. 

 The nuclei are small, and at a certain stage in their develop- 

 ment resemble those of the flagellated cells, but they undergo 

 a process of change which causes them to have a slightly 

 diff'erent appearance in diff'erent larvae (figs. 5 «, b, and c, 7 

 and 7 a, 11, 11 a, 12, 13, 13 a and b \ g. c). 



The composition of the inner mass may be very diff'erent in 

 different larvae as regards the relative quantities of the cell 

 elements above enumerated. Four main types of larvae may 

 be distinguished, which are connected by transitions, but which 



