CHAPTEE VI. 



DIFFERENTIATIONS BETWEEN THE OUTER AND INNEK 

 TISSUES OF ANIMALS. 



§ 287. What was said respecting the primary phj'siological 

 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 betwee]i 

 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 (heir 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 themselvef, 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. J^nd what is 

 here a temporary character answering to a temporary 

 definiteness of conditions, is in the Infusoria a constant 

 c laracter, answering definite conditions that are constant. 

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

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

 the indurated substance being not separable from the sub- 

 stance inclosed, but passing into it insensibly. 



