CELL THEORIES. 129 
layers, of which the outer and the inner may be 
said to be opposed to the middle one, in respect 
that those become epithelial, while this becomes 
the source of other tissues. Further, the important 
observation by His, of the abundant cell prolifer- 
ation at the circumference of the area vasculosa, 
and of the intrusion of the elements so formed into 
the interior of the embryo, must be kept in mind 
as giving rise in early embryology to a primary 
division of the corpuscular masses into centrifugal 
and centripetal. That His is right in his view that 
the whole skeleton and connective tissues, as well 
as the blood and _ blood-vessels, are derived from 
the centripetal group I cannot believe, because 
it appears to me that Remak’s view of the origin 
of the skeleton corresponds much better than that 
of His with what may be seen in _ transverse 
sections of embryos; but it does seem very pos- 
sible that the centripetal development, being the 
source of the blood and blood-vessels, furnishes 
not only nourishment to them but corpuscles which 
conjugate with all those of the centrifugal mass. 
I throw this out as a suggestion, and it will not, 
perhaps, be considered a very wild one, when this 
remark of Stricker’s is remembered :—‘ Were any 
one to maintain that the migrating cells are con- 
jugation organisms, no stronger objection could 
I 
