296 A. A. W. HUBRECHT. 
knob of the blastocyst overcaps the embryonic epiblast, after 
this has entered upon its independent development. It is 
marked @7. (a) in the figures of Pl. XVI, where a comparison of 
the figs. 15—17 with figs. 18—20 will give a general, a compari- 
son of figs. 15 a and 17 a with fig. 20 6, a more detailed idea of 
the changes which this part undergoes during the early stages 
up to the appearance of the mesoblast. At all events it thins. 
out considerably. It should once more be noted that also the 
attachment of the embryonic epiblast to the outer wall, which 
by the nature of the case is massive at the beginning (figs. 15 
—17), thins out as the epiblast of the germinal area increases in 
size. For a time the epiblast of the germinal area may be said 
to be suspended to the wall of the blastocyst by a thin circular 
membrane (ep.’ in fig. 20 6, P]. XVI). The circular attachment 
of this membrane extends centripetally (as described above), 
when the somatic amnion folds are being formed and the sus- 
pension just mentioned ceases at the moment the amuion is 
completed. 
A comparison of figs. 17 @ and 208 on Pl. XVI with figs. 51 
and 52 on Pl. XXV will be useful fully to understand this phe- 
nomenon. Of the different portions of the blastocyst which we 
have hitherto noticed and described in their several ontogenetic 
phases, those that contribute to the formation of the ger- 
minal area and of the embryo shall not be any further noticed 
in this memoir. Nor will I here describe the appearance and 
the development of the mesoblast. 
But we will now turn to a full description of the morpho- 
logical details that are connected with the physiological process 
of the nutrition of the embryo. We have followed the embryo 
up to the stage of the didermic blastocyst. We know that after 
the appearance of the third layer two extra-embryonic regions 
are soon formed which are more especially entrusted with the 
providing of food and oxygen to the growing embryo, viz. the 
vascular areas on the yolk-sac and on the allantois, respec- 
tively connected with the embryo by the vitelline and the 
allantoic vessels. It has been very generally accepted that 
only after the appearance of one or of both of these vascular 
