428 ARTHUR ROBINSON. 
latter hypothetical class ; and this probability is strengthened 
by the fact that the tendency has been transmitted through the 
Protamniota to the Sauropsida, for in the latter division of 
the Vertebrata the segmentation cavity is distinguishable both 
in Lacertilia and Aves. In the Lacertilia it is represented by 
the space which in Hoffmann’s account of their development 
is described in the following words:—‘“ Alle Zellen sind mit 
Dotterk6rmchen sehr stark gefullt. Die obersten bilden 
schon ein eingeschlossenes Blatt, welches nur eine Schicht dich 
ist, und durch einen kleinen, aber deutlichen Zwischeuraum 
vom dem darunter liegenden getrennt ist” (22, p. 1878). 
And again, in the account of a later stage, “‘ An die peripherie 
liegen sie [the hypoblast-cells] in mehren schichten dicht 
aneinander gefiigt und bilden dort den sogennanten Keimwall 
hier liegt der Epiblast unmittelbar dem MHypoblast auf, 
wahrend mehr centralwarts beide Keimblatter durch einen 
deutlichen Zwischenraum vom einander getreunt sind” (22, 
p. 1881). In birds it has been recognised by Duval (9) as a 
small transitory cavity ; and in both Lacertilia and Aves the 
archenteron appears excentrically situated within the hypo- 
blast, its ventral wall, which is enormously hypertrophied, 
forming the yolk-mass. 
6. As the tendency to the production of the above phe- 
nomena, so characteristic of the development of the lower 
Vertebrata, has been transmitted through the Protamniota 
to one of the great branches, the Sauropsida, which has ex- 
tended from it, a priori it might be expected that a similar 
tendency would be transmitted to the only other branch of 
the parent stem—that is, to the Mammalia; and we actually 
find in the rat, the mouse, and the hedgehog, that both the 
segmentation cavity and the archenteron appear as separate 
and distinct cavities, the former of which disappears coinci- 
dently with the formation and extension of the latter, which 
is developed amidst the hypoblast as in the lower Vertebrata. 
In the rabbit and the bat only one cavity appears ; this cavity 
ultimately becomes separated into two parts, the yolk-sac and 
the enteric canal, and presumably, therefore, it corresponds 
