KMBRVOI.OGY OF CLEPSINE. 303 



\'V itli this process " a change iu the character of the two cell- 

 layers goes hand in hand. The cells of the outer layer become 

 larger . . . quite clear and transparent. Those of the inner 

 layer, on the other hand, do not grow, but become darker, 

 yellowish, and finely granular." All this is in harmony with the 

 opinion that the difterentiation of entoderm and ectoderm was 

 originally the result rather than the cause of invagination. That 

 indiscernible differences may have existed in the character of the 

 two cell-layers before invagination is not at all improbable, and 

 this M'ould present no difficulty. 



I have already called attention to the fact that cleavage may 

 proceed in such a manner as to produce no proper blastoccel; 

 but in most cases such a cavity arises as a necessary accompani- 

 ment of the cleavage. Starting with atypical Blastuhi it is not 

 necessary to assume that unequal growth would always result in 

 invagination; but it is easy to see that the more rapid growtli 

 of one hemisphere, accompanied perhaps by an absorption of 

 the blastocoelic fluid, might lead to a double-walled Gastrula. 

 The uidike conditions into which the two hemispheres would 

 tlius be brought would necessarily result in a physiological as 

 well as morphological differentiation. Such an invaginatory 

 mode of growth would act as an economy of space, and the 

 advantage thus offered would afford an opportunity for the opera- 

 tion of Natural Selection. According to this view the blasto- 

 pore, as Lankestcr has stated, would be ''simply the necessary 

 accompaniment of the invagination.^^ Whether it remained 

 open and functioned as a mouth and anus, or closed up, need not 

 here be discussed. 



The transition to the delaminate development is not of course, 

 on one hypothesis more than the other, effected by intermediate 

 forms. The invagination once established, the consequent 

 ditferentiations would become hereditary and thus render it pos- 

 sible for " the segregation of deric molecules from enteric 

 molecules to take place at an earlier point in the embryonic 

 development than that (namely, the blastula-stage), at which the 

 direct adaptative causes could come into operation. ^^ The 

 hereditary difi'erentiation carried to the extreme would manifest 

 itself in the egg even before cleavage, and we should thus 

 arrive at conditions represented in the egg of Geryonia. The 

 cleavage would result as before in a Blastula, and the first tan- 

 gential cleavage (the direction of the cleavage probably being 

 determined by the heriditarily acquired concentric arrangement 

 of the ectoplasm and endoplasm) would result in the delami- 

 nate Gastrula. 



According to this view the phenomenon of polarity, so uni- 

 versally exhibited in eggs, may with some plausibility be regarded 



