CHAPTER VI. 



DIFFERENTIATIONS BETWEEN THE OUTER AND INNER 

 TISSUES OF ANIMALS. 



§287. What was said respecting the primary physiological 

 differentiation in plants, applies with little beyond change of 

 terms to animals. Among Protozoa, as among Protophyta, 

 the first definite contrast of parts is that between outside 

 and inside. The speck of jelly or sarcode which appears 

 to constitute the simplest animal, proves, on closer examina- 

 tion, to be a mass of substance containing a nucleus — a 

 periplast in the midst of which there is a minute endoplast, 

 consisting of a spherical membrane and its contents. 



This parallel, only just traceable among these Rhizopods, 

 which are perpetually changing the distribution of their outer 

 substance, becomes at once marked in those higher Protozoa 

 which have fixed shapes, and maintain constant relations 

 between their surfaces and their environments. Indeed the 

 Rhizopods themselves, on passing into a state of quiescence 

 in which the relations of outer and inner parts are fixed, 

 become encysted: there is formed a hardened outer coat 

 different from the matter which it contains. And what is 

 here a temporary character answering to a temporary definite- 

 ness of conditions, is in the Infusoria a constant character, 

 answering to definite conditions that are constant. Each of 

 these minute creatures, though not coated by a distinct 

 membrane, has an outer layer of excreted substance forming 

 a delicate cuticle. 



§ 288. The early establishment of this primary contrast of 



