288 A, A. W. HUBRECHT. 
epiblast are seen to contain maternal blood, which passes into 
them along channels that will be more fully described in the 
next chapter. The size of the blastocyst is then 0:22 mm. 
The further didermic stages are characterised by a very 
rapid growth, in consequence of which the temporary thickened 
wall of the blastocyst is thinned out to a unicellular layer, with 
strong and numerous villiform processes—the latter the rem- 
nants of what were, in the stage of figs. 8 and 39, the inter- 
vening columns of tissue between two lacune. This pheno- 
menon is best understood by a glance at Pl. XVI, on which 
the blastocysts were drawn with the same power. The critical 
moment here noticed lies between figs. 14 and 15. 
The histological details, which are characteristic for the 
gradual passage from the stage of figs. 11 and 14 to the later 
stages, cannot be gathered from Pl. XVI, but must be studied 
on Pls. XX-—-XXVI. As this whole process is of the highest 
importance for the true interpretation of all the phenomena of 
the hedgehog’s placentation, both omphaloidean and allantoi- 
dean, we will later on return to it more fully. 
We must first give an account of what becomes of the 
thickened knob at one pole of the blastocyst, and secondly, we 
must trace the further development of the hypoblastic cell- 
mass. The thickened knob in question is the spot where the 
germinal area will make its appearauce, which in later stages 
(cf. diagrams of Pl. XVIII and XIX) is also at the pole dia- 
metrically opposite to the mesometrium. The development of the 
germinal area does not come about by a gradual spreading out 
of this thickened polar portion, but by the more unexpected 
splitting off of the deeper bulging portion of this epiblastic 
polar knob from the more superficial portion. Figs. 14—18, 
15 a, 17 a, and 20 d indicate the manner in which this takes 
place. A small break in the middle of the projecting knob is 
the first indication. This increases in size, leaving a space 
between the stratified deeper part, which becomes the em- 
bryonic epiblast, and the more superficial layers. Also, for this 
phenomenon, a glance at the figures on Pl. XVI will be more 
instructive than a long verbal exposition. When once the 
