76 J. W. JENKINSON. 



the outside the medullary plate, which thus lies on what was 

 originally the vegetative pole of the egg (a somewhat awk- 

 ward fact for such hypotheses as Minot's). This second 

 process appears to be represented in the higher Vertebrata 

 by the primitive streak ; the first (in the Sauropsida at 

 any rate) by the growth of the epiblast of the blastoderm. 

 These are facts which any one may observe for himself in 

 the frog, and which, taken in conjunction with Brauer's 

 remarkable and brilliant work on Hypogeophis, do much to 

 remove Lwoff's (21) original generalisation from the region 

 of pure hypothesis. 



It would seem, therefore, that a knowledge of the axes of 

 the ovum during segmentation is necessary before it is 

 possible to speak of gastrulation at all; aud it is just this 

 knowledge which, for Mammalia, we do not possess. More 

 than that, Assheton was unable to confirm in the rabbit van 

 Beneden's statements as to the inclusion of cells derived from 

 one of the first two blastomeres in a layer derived from the 

 other, though Duval has, indeed, reasserted this for the bat. 

 I therefore think it preferable to avoid the term gastrulation 

 altogether, whether applied to the formation of the blasto- 

 cyst, or to the primitive streak; and not even to use the 

 alternative of hypergastrulation suggested by Seleuka to get 

 over the original difficulty of the inclusion of epiblast as well 

 as hypoblast in the inner mass. 



If it is necessary to employ any term at all to express the 

 processes which result in the formation of the blastocyst, I 

 should prefer to speak of a very early development of the 

 amnion (due to whatever causes) in placental mammals. 



This is an actual ontogenetic fact in Erinaceus, Pteropus, 

 and Cavia ; and as it is a stage through which the embryos 

 of all Placentalia pass, I think it is justifiable to believe that 

 it is, for them, the more primitive one. But that this 

 points to the direct descent of the Mammalia from a vivi- 

 parous amphibian aucestor, and to the secondary oviparity, 

 and formation of the amnion by folds in the Sauropsida, as 

 Hubrecht has suggested, is a hypothesis on which I will uot 



