('hai-los-Sedgwick :\Iinot 249 



the oiitddei'iii ) which st"i\os to secure tlie iinphiiitation of the maininaliaii 

 ovum in tlie walls of the uterus. In my "' Text-Book of Eml)ryology " 

 1 have tigured these eells'l'rom the human ovum and ai)i)lied to them 

 the term trophohlast, hut as Professor Huhrecht. who introduced this 

 last term into science, luis ol)jected to this application of it, it has heen 

 necessai-y to inti'oduce a new term, hence the designation trophoderm. 

 It corres])on(ls in lai-ge part, perhaps wholly, to that which Duval desig- 

 nated as the ectoplacenta. It is tlie first tissue in tlie manunals to ho 

 distinctly differentiated. The cells hy their large size, distinct bound- 

 aries, and characteristic nuclei, are readily distinguished from any other 

 cells existing in the embryo at the time the trophoderm is differentiated. 

 Very soon after the development of the trophodermic cells, a large part of 

 them begin to complete their cytomorphosis by undergoing degeneration 

 and resorption. By their disappearance, as I have elsewhere pointed out, 

 the intervillous spaces arise. The trophoderm therefore is not only the 

 earliest tissue to be specialized in the development of mammals, but also 

 the earliest tissue to absolutely complete its cytomorphosis. 



Another striking illustration of the cytomorphic cycle with its phases 

 of differentiation, degeneration, and disappearance of cells is offered 

 to us by the blood corpuscles. The first blood cor])usclcs are cells with a 

 minimum amount of ])rotoplasm. The cells then proceed togrow, and 

 as they grow, differentiate themselves in part at least, into red blood 

 glo1)ules. In mamnuils there follows the stage, degenerative in character, 

 by which the nucleus of these red blood corpuscles disappears. The man- 

 ner of its disa]ipcarance is, to be sure, still perhaps a matter of debate, 

 but for as for the moment is of minor importance. After the degen- 

 eration or disappearance of the nucleus, the blood corpuscles are destroyed 

 and, having completed their cytomorphosis, are replaced by fresh ones. 



A third admirable illustration is offered ns by cartilage, and a fourth 

 bv bone. In cartilage we see at first a differentiation of simple mesen- 

 chvmal cells which then enlarge, becoming the characteristic cartilage 

 cells. When ossification of the cartilage occurs we can easily follow the 

 hypertrophic degeneration of these same cartilage cells, which has been 

 so much studied that good accounts of the enlargement and breaking down 

 of these cells preliminary to the ingrowth of the osteogenetic tissue can be 

 found in all the better text-books of histology; but 1 regret to say I do 

 not recall any text-hook either of anatomy, histology, or embryology, 

 which ]>oints out the fact that this succession of changes in cartilage cells 

 is a typical and almost perfect illustration of cytomorphosis. Almost the 

 same can be said <>r hduc. for in the formation of this tissue also we have 

 first, the difl'ei-ciit iatiiui of the mesenchymal cells into osteoblasts, which 



