392 RICHARD EVANS. 



segmentation^ while the embryo is in the maternal follicle, all 

 the cells possess the same characters ; but between the above 

 stage and the time of hatching, the cells at the surface become 

 ditFerentiated into flagellated cells externally, and cells with 

 granular nuclei beneath, while the cells in the interior of the 

 solid posterior part of the inner mass seem to be arrested in 

 their development. As the results of these changes we obtain 

 the larva which has been described above as type A. It has 

 a flagellated layer at the surface, and cells with granular nuclei 

 beneath and in the neighbourhood of the larval cavity. The 

 posterior end, however, is occupied by the large cells with 

 vesicular nuclei, which represent both morphologically and 

 physiologically a number of unmodified blastomeres. After 

 this stage has been attained, further differentiation seems 

 to take place in the interior, while the flagellated cells, situated 

 as a layer completely surrounding the larva at the surface, 

 are, for a while, retarded in their development. 



The changes in the interior consist in the further differen- 

 tiation of the cells with vesicular nuclei which have, as above 

 described, retained the characters of blastomeres, but which 

 no longer give rise to flagellated cells on the exterior as well 

 as to cells with granular nuclei. The place of the flagellated 

 cells is taken now by the individual elements of the cell groups. 

 Thus another argument in favour of the homology of the small 

 incompletely separated cells, seen in type B, with the flagel- 

 lated cells, is furnished by the fact that, when the cells with 

 vesicular nuclei cease to give rise by differentiation to flagel- 

 lated cells, they begin to give rise to cell groups. This is an 

 important point when taken, not l)y itself, but in conjunction 

 with the arguments already put forward, based upon the identity 

 in size of the individual elements of the cell groups with the 

 flagellated cells, and the similarity of their nuclei. 



There results, as the outcome of these internal changes, the 

 larva described above as type B, from which type C is derived 

 by further differentiation along the same line. The multi- 

 nucleated masses of cytoplasm in the former divide into as 

 many coipuscles of cytoplasm as there are nuclei in the whole 



