DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 413 
formation of the hind gut ; but when the caudal portion of the 
gut is well developed the relations of the floor of the neural tube 
to the mesoblast, and of the chorda to the termination of the 
gut and the mesoblast, are the same as those found at the an- 
terior end of the primitive streak at the eleventh day. It 
seems probable, therefore, that the hind gut is formed from the 
hypoblast of the anterior third of the primitive streak, and 
that the anterior portion of the streak forms the growing 
point of the tail, in which case the fate of the primitive streak 
of mammals is exactly similar to the fate of the primitive 
streak of Rana temporaria (40). 
The Segmentation Cavity and the Didermic 
Blastocyst. 
It is during the period which precedes the completion of the 
didermic condition that peculiarities appear in the mamma- 
lian ovum which render its comparison with the ova of other 
Vertebrates a matter of some difficulty. 
A complete didermic vesicle—that is, a vesicle with a double 
wall entirely surrounding the cavity, and with no interveniug 
stratum anywhere present between the two layers—exists both 
amongst the lower and the higher Vertebrata: it is found, e. g., 
in the Amphioxus, Cyclostomes, Ganoids, Amphibians, and in 
Mammals. In a large number of Vertebrates, however, a 
third layer appears before the second layer is completed, and 
it is both customary and convenient to speak of a developing 
vesicle as didermic, even when only part of its wall is formed 
by two of the primitive germinal layers; a didermic stage 
may, therefore, be considered to be universal in the develop- 
ment of the Vertebrate ovum. The mode of formation ofthe 
didermic vesicle and the relations which its two layers bear 
to each other vary considerably. 
In Amphioxus (15, 28) the segmentation of the ovum is 
followed by the formation of a monodermic vesicle with a 
central cavity, the segmentation cavity ; and the didermic con- 
dition is produced by the invagination of a portion of the wall 
of the vesicle ; at the same time the segmentation cavity is 
