DEVELUPMENT OF GERMINAL LAYERS IN MAMMALS. 419 
phenomena observable in the rat and mouse do not substantiate 
this inference; on the contrary, they very definitely indicate 
the reverse conclusion, for a portion of the cavity becomes the 
true enteric canal, and from its walls both notochord and meso- 
blast are formed, as in lower Vertebrates. But although, in 
the main features of the early stages of development, the ova 
of the rat, mouse, and hedgehog correspond closely with the 
ova of the Lacertilia and the lower Vertebrata, in the ova of 
the majority of the Mammalia which have been investigated, 
peculiarities in the formation of the didermic vesicle are de- 
scribed which have hitherto received no satisfactory explana- 
tion, and there is room for difference of opinion concerning 
the nature of the cavity contained within the two primitive 
layers. 
In the cat there seems reason to believe, from Professor 
Schafer’s account of an early ovum (48), that the processes by 
which the didermic vesicle is formed correspond with those 
which obtain in the hedgehog, but we have no definite proof 
that this is the case. Bonnet’s account of the sheep does not 
extend to the stages which precede the completion of the 
didermic vesicle. Precise accounts have been given of the de- 
velopmental features which precede the completion of the 
didermic stage in the opossum (46), rabbit (2, 27), mole (17), 
shrew (23), bat (4), and the guinea-pig (45). In the ovaof all 
these mammals, according to the descriptions given, the cavity 
which afterwards becomes the cavity of the yolk-sac lies at first 
between the epiblast and the hypoblast, and it is afterwards 
enclosed by the hypoblast, which extends round the inner sur- 
face of the epiblastic wall of the blastodermic vesicle ; in other 
words, a cavity which lies outside the hypoblast, between it 
and the epiblast, and which is therefore in the position of the 
segmentation cavity of the rat, mouse, hedgehog, and the 
lower Vertebrates, becomes converted into the cavity of the 
yolk-sac. This is a distinguishing feature, and, so far as I am 
aware, no connecting link has been found which can be looked 
upon as uniting the ova in which the epiblast grows round the 
hypoblast to the ova in which the hypoblast is said to grow 
