THE EAKLY DEVELOPMENT OE THH MAIiSEPIALIA. 71 



over an arc of about 60° from the upper pole in all directions." 

 These "straggling*" cells^ as Asshetou terms them^ as well as 

 the innermost cells of the now flattened inner cell-mass, are 

 regarded as hypoblastic and the outermost cells of the same 

 as epiblastic (embryonic epiblast). "The hypoblast, as a 

 perfectly definite layer, is formed by the time the blasto- 

 dermic vesicle measures '5 mm. in diameter, that is, about the 

 102iid hour after coition. It is not, however, as yet by any 

 means a continuous membrane ; it is a network or fenestrated 

 membrane. For this reason, in. section it appears to be 

 represented by isolated cells lying beneath the embryonic 

 disc (v. fig. 29, Hy.)" (cf. Dasyurus). In considering the 

 question how the peripherally situated ("straggling") ento- 

 dermai cells, which are undoubtedly derived from the inner 

 cell-mass, " apparentl}' wander round the inside of the blasto- 

 dermic vesicle," he reaches the conclusion that this is not the 

 result of amoeboid activity or growth "in the sense of migra- 

 tion " on the part of these cells, but " is only an apparent 

 growtli round produced by the more rapid growth of a 

 zone of the [trophoblastic] wall of the vesicle immediately 

 surrounding the embryonic disc, in which zone the marginal 

 cells of the inner mass lie." He is unable to find any 

 evidence of the production of pseudopodial processes by 

 these peripheral entodermal cells, the majority of them 

 appearing at first to be quite isolated from each other and 

 approximately spherical. " Certain of the cells here and 

 there are connected by threads of protoplasm, but this, I 

 think, is not a sign of pseudojDodic activity, but merely 

 indicates the final stage in division between the two cells." 

 By the sixth day the hypoblast of the embryonic disc has 

 assumed the form of a continuous membrane, composed of 

 completely flattened cells, Avhilst the peripheral hypoblast 

 cells have become more numerous, and "many of them, 

 possibly all of them, are now undoubtedly connected by more 

 or less fine protoplasmic threads." Such, in brief, is 

 Assheton's account of the early history of the entoderm in 

 the rabbit; it presents obvious points of agreement with my 



