MORPHOLOGICAL DIFFERENTIA 



47 



the most characteristic physiological peculiarity of the plant 

 is its power of manufacturing protein from chemical com- 

 pounds of a less complex nan 



The most characteristic morphological peculiarity of tl,, 

 animal is the absence of any such cellulose iim-v 

 most characteristic physiological peculi i.al U 



its want of power to manufacture protein out of simpler 

 compounds. 



The great majority of living things are at once referable 

 to one of the two categories thus defined ; but th. r, :IM - some 

 in which the presence of one or other chara< mark 



cannot be ascertained, and others which appear at difT< 

 periods of their existence to belong to different categories. 



II. THE MORPHOLOGICAL DIFFERENTIATION OF 



The simplest form of animal life imaginable would be a 

 protoplasmic body, devoid of motility, maintaining it. self by 

 the ingestion of such pioteinaceous, fatty, amyloid, and min- 

 eral matters as might be brought into contact with it l>v ex- 

 ternal agencies ; and increasing by simple extension of its 

 mass. But no animal of this degree of simplicity is kno\\n 

 to exist. The very humblest animals with which we are ac- 

 quainted exhibit contractility, and not only increase in size, 

 but, as they grow, divide, and thus undergo multiplier 

 In the simplest known animals the Protozoa the proto- 

 plasmic substance of the body docs not hrromr ilitli n ntiated 

 into discrete nucleated masses or cells, which l.y their n 

 morphosis give rise to the different tissues of which the adult 

 body is composed. And, in the lowest of the Protozoa, the 

 body has neither a constant form nor any further <1 

 of parts than a greater density of the peripheral, as < 

 pared with the central, part of the protoplasm. The first 

 steps in complication are the appearance of one or more 

 rhythmically contractile vacuoles, such as are found in some 

 of the lower plants ; and the segregation of part of the in- 



1 No analysis of the substance composing the cysts in which so many of the 

 Protozoa inclose themselves temporarily has yet been made. But i- 

 probable that it may he analogous to cftitin /'ami. it' s>. it is worthy "' remark 

 that, though chitin 'is a nitron-nous body, it readily y it-Ids a Mil-tanc* appar- 

 ently identical with cellulose when heated with the double 

 OOpper and ammonia. It is possible, then-tore, that tin- .iitfcrence Del* Mil 

 the chitinous investment of an animal and the eell 'tm-nt of a plant 



y depend upon the proportion of nitrogenous matter which ia present in 



h case in addition to the chitin. 



ma 

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