DEVELOPMENT OF GERMINAL LAYERS IN MAMMATS. 369 
Observations upon the Development of the Seg- 
mentation Cavity, the Archenteron, the Ger- 
minal Layers, and the Amnion in Mammals. 
By 
Arthur Robinson, M.D., 
Senior Demonstrator of Anatomy at the Owens College, Manchester. 
With Plates XXIII, XXIV, XXV, XXVI, and XXVII. 
SUMMARY. 
A.—General description of the development of the ova of the rat and mouse 
up to the period of completion of the blastodermic vesicle, and com- 
parison with the results obtained by Professors Fraser, Duval, and 
Selenka. 
Results.—There is a segmentation cavity, which is not the blasto- 
dermic cavity. The segmentation cavity disappears simultaneously 
with the appearance of the archenteron. The archenteron appears 
amidst the hypoblast. The young ovum consists principally of 
hypoblast, which becomes vacuolated to form the cavity of the yolk-sac. 
The cavity of the yolk-sac is never bounded by epiblast alone. The 
epiblast extends over the outer surface of the hypoblast, not the 
hypoblast over the inner surface of the epiblast. The hypoblast never 
becomes entirely surrounded by epiblast. 
B.—Description of the formation of the mesoblast. 
Results.—The mesoblast is formed partly from the peristomal 
cells in the region of the primitive streak, partly from the embryonic 
hypoblast, and partly from the extra-embryonic hypoblast. Meso- 
blastic formation commences at the posterior end of the embryonic 
area, not anteriorly, as in the hedgehog. In the embryonic area the 
pericardial mesoblast is the last formed. 
C.—Description of the formation of the chorda dorsalis. 
Results.—The chorda is formed entirely from the primitive 
hypoblast, to which it last remains adherent at the dorsal end of the 
bucco-pharyngeal membrane and the anterior end of the primitive 
streak. There is no ‘“ Kopffortsatz” of the primitive streak. 
