264 



THE CELL-THEORY 



of those properties, which are called vital forces. What, then, are 

 these cells ? it may be asked ;— what is the meaning of the unques- 

 tionable fact that the first indication of vitality, in the higher organisms 

 at any rate, is the assumption of the cellular structure ? 



In answering these questions, we would first draw attention to the 

 definition of the nature of development in general, first clearly enun- 

 ciated by Von Baer. " The history of development," he says, " is the 

 history of a gradually increasing differentiation of that which was at 

 first homogeneous." The yelk is homogeneous ; the blastoderma is a 

 portion of it which becomes different from the rest, as the result of the 

 operation of the laws of growth ; the blastoderma, again, comparatively 

 homogeneous, becomes differentiated into two or more layers ; the 

 layers, originally identical throughout, set up different actions in their 

 various parts, and are differentiated into dorsal and visceral plates, 

 chorda dorsalis and bodies of vertebrae, &c., &c. No one, however, 

 imagines that there is any causal connexion between these successive 

 morphological states. No one has dreamt of explaining the develop- 

 ment of the dorsal and visceral plates by blastodermic force, nor that 

 of the vertebrae by chorda-dorsalic force. On the other hand, all these 

 states are considered, and justly, to result from the operation of some 

 common determining power, apart from them all — to be, in fact, the 

 modes of manifestation of that power. 



Now, why should we not extend this view to histology, which, as 

 we have explained, is only ultimate morphology ? As the whole animal 

 is the result of the differentiation of a structureless yelk, so is every 



I tissue the result of the differentiation of a structureless blastema — the 

 first step in that differentiation being the separation of the blastema 



' into endoplast and periplast, or the formation of what is called a 

 ' nucleated cell." ^ Then, just as in the development of the embryo, 

 when the blastodermic membrane is once formed, new organs are not 

 developed in other parts of the yelk, but proceed wholly from the 

 differentiation of the blastoderm, — so histologically, the " nucleated 

 cell," the periplast with its endoplast, once formed, further development 

 takes place by their growth and differentiation into new endoplasts 

 and periplasts. The further change into a special tissue, of course, 

 succeeds and results from this primary differentiation, as we have seen 

 the bodies of the vertebrae succeed the chorda dorsalis ; but is there 

 any more reason for supposing a causal connexion between the one 

 pair of phenomena than between the other? The cellular structure 

 precedes the special structure ; but is the latter, therefore, the result of 



' Compare Reichert, p. 35. 



