2")0 Genetic Initerpretatioiis in tlie Domain of Anatomy 



are always of larger dimensions than the cells fi-om Avliicli they arise; 

 and after these osteoblasts have lieeome bone cells they cease their devel- 

 opment and apparently degenerate. I have to say apparently . because, so 

 far as I know, the fate of the bone corpuscle has not been ascertained 

 with certainty. We risk but little, however, in asserting that the bone 

 cells also offer an instance of a normal, complete cytomorphosis. 



As a fifth and last illustration, let us choose the epidermis, in which 

 Ave have a distinct type of differentiation. In the basal layer are the cells, 

 which divide and produce, according to our present notions, all of the cells 

 of the epidermis. When the basal layer cells divide, however, some of 

 them only, pass immediately through further cytomorphic changes in 

 order to make first, the cells of the mucous layer, and later, by under- 

 going cornification, to constitute the horny layer. Others of the basal 

 cells remain members of the basal layer and continue to proliferate. We 

 thus see the progeny of the original basal cells divided into two classes: 

 the cells of one class pass on in their development, the others retain their 

 ancestral type. In the epidermal cells we observe as in other instances 

 of cytomorphosis, first the enlargement and differentiation of the cells, 

 here occurring in the mucous layer, and later their degeneration or corni- 

 fication followed by their necrosis and destruction. 



It would be easy to multiply these illustrations. All of you could 

 supply more. That which I would urge upon your consideration is the 

 value of the cytomorphic interpretation in explaining the origin and 

 differentiation of tissues in the light of the broadest principle of cellular 

 development which we have up to the present time been able to establish. 



I will now ask you to consider certain possible genetic classifications. 

 The most fundamental and important of these seems to me to be that of 

 tissues and of organs according to the germ layers from which they arise. 

 This classification was made the basis of his entire course of lectures upon 

 animal morphology by Professor Carl Semper, the Wiirzburg zoologist, 

 under Avhom I had the pleasure of studying in 1875-76. It is not merely 

 very practical and advantageous alike to teacher and pupil, but is also 

 the only thoroughly scientific classification of structures and organs which 

 we can adopt. No other classification should, in my judgment, be seri- 

 ously considered. So firmly do I hold this conviction that I greatly 

 deplore the fact that our text-books of histology are not written upon 

 an embryological basis, the lack of which deprives them of much of the 

 scientific character and value they ought to have. 



As our knowledge of the development from the germ layers has grown, 

 we have learned with ever-increasing certainty that each germ layer 

 has its specific role to pla}^ Each germ layer produces its own specific 



