114 MCMURRICH. [Vol. XI. 



due to these same forces. We must assume, accordingly, that 

 intrinsic forces reside in all ova, though they may be over- 

 shadowed by external influences in some cases. In fact this 

 idea may be carried further, and it may possibly be that the 

 action of external forces may be sufficient to interfere with the 

 arrangement of the cells which would result if they were ex- 

 cluded, and the histological differentiation of the ovum be 

 retarded thereby. 



One other point which the study of Isopod cleavage has sug- 

 gested seems to be worthy of notice here. It is connected with 

 the question of the value of cytogeny as a basis for homology, 

 a question which has already been discussed by Wilson ('92), 

 who points out that " cells having precisely the same origin in 

 the cleavage, occupying the same position in the embryo, and 

 placed under the same mechanical conditions, may nevertheless 

 differ fundamentally in morphological significance." A com- 

 parison of the cleavage oi Jaera with that of Aselhis brings sup- 

 port to this view. Thus in the eight-celled stage of the former 

 species (Fig. 9) we find at one pole of the Qgg a cell D, which 

 gives origin to all the vitellophags of later stages ; in the eight- 

 celled stage of Ascllns (Fig. 30) we find an exactly comparable 

 arrangement of the cells, but the cell D has by no means the 

 same morphological significance, inasmuch as it does not con- 

 tain vitellophag elements alone. Similarly mjaem, in the six- 

 teen-celled stage (Fig. 12), there are six cells. A', c, c', and d\ 

 which give rise to the liver-endoderm and mesoderm of later 

 stages, while in Aselhis it will be seen that the same cells, the 

 same, that is, so far as their cytogeny is concerned, are entirely 

 ectodermal. Indeed the difference between the two species 

 may be expressed by stating that the differentiation of .the 

 germ-layers takes place practically at one stage later in the 

 cleavage in Asellus than in Jaej'a. 



The significance of this is by no means certain, though it is 

 interesting to note that an explanation of it may be found 

 in the different size of the two ova. Thus the egg of Asellus 

 is several times the size of that oijaera, and it maybe that the 

 extra amount of yolk acts in retarding the differentiation, or, in 

 other words, a certain spatial relationship of the blastula cells 



