STUDIES IN MAMMALIAN EMBRYOLOGY. 535 



than is done in RabPs original figure. We can identify this 

 layer with the light blue layer overcapping the yolk-cells in 

 fig. 92. 



In how far also in Sauropsida regions might be distin- 

 guished that could be identified with protochordal plate and 

 protochordal wedge in a stricter sense, and in how far an 

 annular mesoblast-producing zone of hypoblast can also here be 

 distinguished, are questions that will have to be reinvestigated 

 very fully. We have not as yet enough data for any definite or 

 exhaustive answer. ^ The chief point is that there is no diflficalty 

 in comparing the diagrammatic stage (fig. 92), which we start 

 from in attempting to interpret the gastrulation of the Mam- 

 malia, with the diagrams for the Sauropsida which Rabl has 

 given. This is all the more important because with respect to 

 the Mammalia, this author, while acknowledging the insuffi- 

 ciency of the data at his disposal, yet inclines the other way, 

 and has expressed himself in favour of van Beneden^s views, 

 which I must dissent from. 



In Sauropsida (1) the great bulk of the yolk ; (2) the parti- 

 cipation of the upper layers in the phenomenon of retarded 

 cleavage (Nachfurchuug), by which new cell-material is added 

 to the embryonic tissues; and (3) the simultaneous appearance 

 (at least in the chick) of hypoblast and mesoblast, are pheno- 

 mena which obscure the early points in contest by which the 

 formation and homology of the layers can be judged. 



In discussing the Mammalia, where, on the contrary, a well- 

 defined didermic stage is indisputably present, we can there- 

 fore not derive much benefit from the diagram that applies to 

 the Sauropsida, and we shall have to fall back upon another 

 hypothetical intermediate stage. However, before doing this, 

 the diagram here given of the phenomena as we actually find 

 them in the Mammalia must first be more closely looked at. 

 It is fig. 95 which represents a diagrammatic longitudinal 

 section of a mammalian blastocyst (Sorex, Talpa, Ovis), 



1 Figs. 53, 56, and GO in Duval's 'Atlas d'Embryologie ' (1SS9) are very 

 suggestive as far as the protochordal plate is concerued ; but I will refraiu 

 from any further discussion at present. 



