DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 371 
satisfactory explanation, for in none of the other Vertebrata 
does the segmentation cavity become surrounded by hypoblast. 
The relationship of the blastodermic cavity to the archen- 
teron is disputed. Van Beneden believes the former cavity to 
be a yolk-space which has no genetic association with the 
primitive alimentary cavity, which is represented in the mam- 
malia by the notochordal canal, from the dorsal wall of which 
the notochord is separated. In opposition to this opinion 
Bonnet (5, 6) and Hubrecht (23) have shown that a portion of 
the blastodermic cavity becomes converted into the enteric 
canal, and that at least a part of the chorda dorsalis is 
developed from the hypoblast which forms the wall of the 
primitive cavity. It must be noted, however, that Kolliker 
(26) and Keibel (25) deny the hypoblastic origin of the chorda, 
and that they look upon this organ as a purely mesoblastic 
structure which is evolved from the ‘‘ Kopffortsatz” of the 
primitive streak. 
Quite recently an attempt to reconcile these opposed con- 
clusions has been made (23), and for this purpose the hypo- 
thesis has been advanced that the hypoblast in the Mammalia 
is formed in two separate portions. One of these, which ulti- 
mately surrounds the blastodermic cavity, becomes precociously 
segregated and separated, and is therefore termed the cceno- 
genetic hypoblast. The second portion, the phylogenetic 
hypoblast, undergoes a modified embolic invagination at a 
later period, part of it appearing as the “ Kopffortsatz ” of the 
primitive streak. The chorda dorsalis is formed partly from 
the coenogenetic hypoblast and partly from the “ Kopffortsatz,” 
and thus entirely from the inner germinal layer, as in the lower 
Vertebrata. 
The mode of formation of the middle germinal layer is also 
a subject of dispute. It is said to arise from the epiblast alone 
(12, 26, 27, 44), or partly from the epiblast and partly from 
the hypoblast (1, 5). There is also difference of opinion with 
regard to the position in which the middle layer is first formed. 
It is said by some to arise only in the posterior part of the 
germinal area in the region of the primitive streak (12, 27). 
