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 Strieker's is remembered : " Were any 

 one to maintain that the migrating cells are con- 

 jugation organisms, no stronger objection could 



