EMBRYOGENETIC CLASSIFICATION 309 



indicate rapidly what are the leading facts with regard to the 

 earlier stages in the development of the different types of tissue, 

 and I must recapitulate matters of a most elementary nature ; 

 nevertheless, if by doing this I can make my argument clear, I 

 trust that I shall be forgiven. 



The earliest stage to be recognized in the development of 

 the fertilized ovum, once it has proceeded to segment, is the 

 production of a morula, in which the blastomeres form a cluster 

 or group of cells of the same order, with almost entire lack of 

 differentiation. Rapidly this gives place to a second stage, in 

 which the component cells arrange themselves into two layers, 

 the epiblast and the hypoblast, so that, at a singularly early 

 stage, the future epiderm and endoderm are recognizable. The 

 next stage to be noted is that the hypoblast, or internal of the 

 two primitive layers, gives rise by proliferation of its cells to a 

 group or mass of cells which now lie intermediate between the 

 primitive epiblast and the hypoblast, and form the " Anlage " 

 of the mesoblast and of the organs derived from that layer, the 

 hypoblast itself still remaining as a distinct lining membrane. 

 What I would here note is that we are perfectly willing to advance 

 thus far and recognize these two primary and the third secondary 

 cell layer. But there, in our appreciation of embryology as bear- 

 ing upon pathology, we have been strangely apt to stop. 



But now, just as the hypoblast gives origin to the mesoblast, 

 so it is perfectly legitimate for us to recognize a similar process 

 on the part of the epiblast ; for the epiblastic cells in the im- 

 mediate neighbourhood of the primitive groove proliferate rapidly, 

 and, in so doing, project in part below the original line of the 

 epiblast, and, being forced inwards, a regular mass of cells is 

 developed, in the central portion of which, around the spinal 

 canal, there still remains evidence of its origin, in the form of 

 a definite epithelial lining. This second portion becomes cut 

 off completely from the superficial epiblast to form the mother 

 tissue of the nervous system. Similarly, the hypoblast gives 

 off a second localized mass of cells to form the notochord. 



Professor Minot has called my attention to the fact that, 

 while possessing special features, the cells forming the notochord 

 throughout retain epithelial characters, that they are grouped 

 together without an intercellular matrix, that no vessels penetrate 

 between them, and that, although in degeneration these cells 



