370 723 ARTHUR ROBINSON. © if hi voi 
D.—Comparison of the ova of the rat and mouse with the ova of other 
mammals and the lower vertebrates. 
Conclusions.—The ova of mammals do not differ essentially from 
the ova of other vertebrates. They do not consist in the early stages 
of an epiblastic vesicle containing an inner mass of epiblast and hypo- 
blast, but of a large hypoblastic mass which supports a small epiblastic 
disc. The ova of mammals present all the characteristic features of 
comparatively large-yolked ova. 
E.—Description of the formation of the amnion and discussion of the rela- 
tion of amnion formation to “ inversion.” 
Conclusions.—There is a pro-amnion in the rat and mouse. 
Pro-amnion formation and “inversion” are distinct processes, and 
“inversion” is not precocious pro-amnion formation. The whole of 
the amnion is formed from the tail-fold. — 
Comparison of amnion formation in-the rat and mouse with amnion 
formation in man and other mammals. 
F.—Description of the formation of the ccelom. 
Results.—The celom commences bilaterally and in the embryonic 
area. The pericardial ccelom is an extension of the embryonic ccelom 
from behind forwards. It does not communicate with the anterior 
portion of the extra-embryonic ccelon. 
Tue formation and the extension of the germinal layers, 
their relation to the segmentation cavity and archenteron, 
and the derivation of the chorda dorsalis in the mammalia are 
developmental problems which have’ initiated many investiga- 
tions. From the results which have hitherto been obtained 
only very general conclusions can be drawn, and upon many 
important points the recorded observations are contradictory. 
This is all the move noteworthy inasmuch as many of the obser- 
vations have been made upon the same kind of animal ; others, 
however, upon animals belonging to widely divergent species. 
_ The blastodermic cavity of the mammalian ovum lies, at 
first, between the epiblast and hypoblast and corresponds in 
position with the segmentation cavity of the lower Vertebrata. 
In the rabbit (2), rat (9,13, 45), mouse (9, 18, 44), guinea-pig 
(45), shrew (23), mole (16), bat (4), and opossum (46) it is 
said to become surrounded by the hypoblast. If this actually 
occurs, then the mammalia are separated from all the other 
Vertebrata by a peculiarity which as yet has received no 
