C Y TOMORPHOSIS. 1 3 



probable that every organ is represented by some region in the ovum itself. In 

 other words, although the organs of course do not exist preformed in the egg, 

 yet the material for them is there and prelocalized. Roux has developed this 

 conception and has compared the egg to a mosaic; each member of the mosaic 

 is assumed to undergo self-differentiation or its predestined development. Com- 

 plete verification of this theory has not been secured, but it has been demonstrated 

 that certain eggs of animals of several invertebrate orders contain substances, 

 which have an exact distribution, and which have a definite fixed relation to adult 

 structures. By putting some of these eggs in a centrifuge these substances may 

 have their distribution artificially changed. In eggs thus altered the substances 

 continue to transform themselves into their predestined structures, which conse- 

 quently appear displaced. By these experiments the mosaic theory has received 

 a limited confirmation. 



At present it is impossible to reconcile the two theories of the constitution 

 of the ovum, but since both are based apparently on facts it seems probable that 

 they will, by wider knowledge, be fused into a single coherent conception. 



Meanwhile the fact of most importance in cytomorphosis is, that the protoplasm 

 of the ovum is undifferentiated and lacks completely any variations of its stnu 

 comparable to those which we observe as characteristic in the cells of adult tis- 

 The potentialities of the ovum, on the other hand, are of course very great. Experi- 

 mental embryology is now endeavoring to ascertain what physiological causes ren- 

 der those potentialities active. From physiological embryology much is to be 

 expected. 



2. Differentiation. This may be defined as a process by which the structure of 

 the cells is modified, so that they become dissimilar by acquiring an organization 

 which adapts them to special functions. The cells which arise during the segmen- 

 tation of the ovum differ but slightly from one another. As development progresses 

 we find the cells change, some in one way, some in another, so that many kinds of 

 cells are produced, but of each kind we find a large number of cells. Each kind 

 of cell may be said, roughly speaking, to form a tissue for itself. Cells of each 

 tissue offer visible peculiarities by which they may be readily distinguished from one 

 another under the microscope. It thus appears that the production of tissues is the 

 main result of differentiation, so that this process of development may be fairly accu- 

 rately defined as equivalent to histogenesis. As to the factors which cause differen- 

 tiation, we have no satisfactory knowledge. We can, at present, only note the changes 

 when they acquire such magnitude as to become microscopically visible. As to the 

 physiological conditions which cause these changes we have almost no conceptions. 

 It is probable that the nucleus has a leading role to play, but our knowledge of 

 this role is too little advanced to permit a profitable discussion of the subject here. 



The actual process of differentiation shows itself both in the protoplasm and in 

 the nucleus of the cell. The changes in the former are the more conspicuous, and 

 therefore the better known. The changes in the nucleus have still to be adequately 



