WEYSSE. — BLASTODERMIC VESICLE OF SU3 SCROFA. 307 



Ignore Hensen's own statement of the case when he says, " Die innere 

 Keimhaut eeht nur iiber das obere Drittheil des Eies ; wenn an audere 

 Stellen die Keimhaut zweischichtig erscheint, so moge man dies aus 

 der Dreliung beim Uebergang von der Flachen- in die Kauteuansicht 

 erklaren." The figure, however, does give the appearance of two 

 layers at the region under discussion. While such observations as 

 those of Heape ('83) and of Schafer ('76) would seem to point to two 

 distinct methods of the extension of the entoderm in Mammalia, I can- 

 not affirm with certainty that either occurs in the pig ; what evidence I 

 have, I am inclined to interpret in support of the phenomenon as it is 

 said to occur in the rabbit, the mole, etc. ; i. e. as a pi-ocess of marginal 

 growth from the inner mass of cells of the germinal disk. 



The process of entoderm formation from the inner mass of cells 

 would seem to be primarily, then, a separation of certain cells from 

 the general group, and I cannot help drawing attention here to the 

 fact that there may be an homology between this process and the 

 method of entoderm formation described by Robinson and Assheton 

 ('91) in the frog; furthermore, the formation of the didermic stage in 

 the rat, as interpreted by Robinson ('92), is comparable with this, for 

 he finds first a mass of irregular cells, in which a cavity develops sep- 

 arating a single layer of cells at one pole from a mass of cells at the 

 other ; the single layer he considers ectoderm, and the cells at the 

 opposite pole the entoderm, or, as he calls them, epiblast and hypoblast 

 respectively. I am not, however, sure in the light of the results of 

 Selenka ('83) and Duval ('91), that this interpretation is correct. 



Before leaving the general consideration of the blastodermic vesicle, 

 there are two or three other points to which I wish to refer. Schafer 

 ('76) found in the embryo of the cat a clearly defined non-cellular 

 membrane over the outer surfiice of the entoderm in the region of the 

 germinal disk ; this he calls the " membrana limitans hypoblastica," 

 and compares it to the " membrana prima" desciibed by Hensen ('76) 

 in the first part of his paper on the rabbit (see Hensen, Fig. 19, Taf. 

 IX.). In the second part of his paper, Hensen figures the membrane 

 as in contact with the entoderm in the region of the primitive streak 

 only, and as then passing over the dorsal side of the mesodermal 

 somites, and coming in contact with the ectoderm laterally (see his 

 Fig. 37, Taf. X.). In Hensen's eai'lier figure, and in Schafer's, this 

 membrane is clearly an entodermal structure, and I hold it comparable 

 to the membrane which I have found between the ectoderm and the 

 entoderm of the germinal disk, which leaves the ectoderm at the margin 

 of the disk and passes off to meet the extra-germinal ectoderm farther 



