CHAPTER YE. 



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 that arises 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 

 defmiteness of conditions, is in the Infusoria a constant 

 character, answering definite conditions that are constant. 

 Each of these minute creatures, though not coated by a dis- 

 tinct membrane, has the outer layer of its sarcode indurated : 

 the indurated substance being not separable from the sub- 

 stance inclosed, but passing into it insensibly. 



