M ON NORMAL AS A 



at which it becomes necessary to classify the products of morbid 

 growth. Now their minute structure affords no adequate basis 

 for classification; we must seek aid from some principle of a 

 broader kind, and such a principle can only be found by 

 attempting, as already suggested, to institute a careful com- 

 parison between the phenomena of morbid and those of normal 

 growth. We must distinguish between the initial and the later 

 stages of normal growth, for it is chiefly among the latter that 

 we find the analogies of which we stand in need. 



The first foundations of every organ are laid by the difie- 



rentiation of elementary parts which are originally equivalent. 



These elementary parts are small round cells, furnished with 



large nuclei ; the blastoderm and a7'ea germmativa are formed 



by a continuous aggregation of these cells. The separation of 



the blastodermic membrane into three layers, which next ensues, 



has recently been subjected to much discussion. So much 



appears certain, that the organs of locomotion and sensation are 



developed from the upper layer; from the under one those of 



respiration and digestion, while the middle layer is destined to 



form the blood-vessels and the connective tissues. His believes, 



however (and his view is to some extent confirmed by Waldei/ev^s 



researches), that from the very first moment of this subdivision 



there exists an absolute contrast or opposition between the upper 



and lower (two outermost) strata of the blastoderm on the one 



hand (tlie organopoietic strata proper, which, taken together, 



form the neuroblast), and the intermediate layer, the Jicemohlast, 



on the other ; the latter being formed, not, as has hitherto been 



supposed, by a splitting of the inferior layer of the blastoderm, but 



by an independent ingrowth from the edges of the discus proli- 



gerus. It would indeed be interesting to find the independent 



character of the blood-vessels and the parenchyma, of the nutriens 



and the nutriendum, shadowed forth even in these earliest stages 



of development. The mutual independence of these two great 



components of the organism is definitively established, when 



the first rudiment of a vascular apparatus, the area vasculosa, 



makes its appearance ; and there can be no doubt that it affects 



the ulterior development of the individual most profoundly. 



From this time forth, no sooner is any new organ differentiated 



from the continuous aggregate of embryonic cells, no sooner do 



we gather from the mode of aggregation of the specific elements 



