Haddon — Note 0)1 the Blastodermic Vesicle of Mammals. 539 



wall, is the seat of a rapid and early proliferation which pushes the 

 embryonic area into the blastodermic vesicle. The underlying em- 

 bryonic area alone enters into the formation of the embryo itself, 

 the epiblastic layer of which consists, apparently from the first, 

 of a single layer of cells. 



In the earliest stage of the rat, figured by Fraser (8), sis days? 

 eight hours after impregnation, the blastodermic vesicle is elon- 

 gated, and its cavity is greatly reduced by the projection of a solid 

 mass of epiblast cells which projects into the cavity of the vesicle ; 

 the inner aspect of these cells is lined by a single layer of hypo- 

 blast. A well-marked layer of rounded cells (covering cells, Deck- 

 enzellen) occurs at the placental pole of the blastodermic vesicle. 

 In twenty- four hours the epiblast forms a single layer of colum- 

 nar cells eu closing a central cavity, the neuramniotic + the false 

 ammion cavity. The Deckenzellen have by this time greatly in- 

 creased in number. 



From the above it is quite clear that the outer cells of the fully 

 segmented mammalian ovum correspond to the epiblast of the area 

 opaca, or yolk-blastoderm, and that the epiblast of the embryo is 

 formed from the upper portion of the inner mass, the lower portion 

 giving rise to the hypoblast. There is still some discrepancy as to 

 the fate of the outer or covering cells which grow over the " blasto- 

 pore " of van Beneden, and which form the flattened upper layer 

 of the rabbit's blastoderm. According to Balfour (rabbit), Heape 

 (rabbit and mole), and Lieberkiihn (dog and mole), this layer 

 enters into the foroiation of the embryonic epiblast ; KoUiker and 

 Eauber believe it disappears in the rabbit ; Kiipifer (Arvicola) and 

 Fraser (rat and mouse) assert that the rounded cells of this layer 

 become attached to the decidua, and do not form any portion of 

 the embryo proper. 



A consideration of these facts has led me to the view that we 

 must recognize in the inner mass a germinal disc perfectly com- 

 parable with that of the fowl during the later stages of segmenta- 

 tion (cf. The Elements of Emhryologi/, by Foster and Balfour, 1883, 

 fig. 8, p. 22), which has sunk into the blastodermic vesicle oioing to the 

 absence of yolk. The outer layer corresponds to those epiblast cells 

 which are gradually enclosing the yolk, the so-called blastopore of 

 van Beneden indicating in an exaggerated manner the distinction 

 between the embryonic and non-embryonic germinal layers. 



