TYPES OF CONSTRUCTION IN THE ANIMAL KINGDOM. 221 



tion, or what wc may call the principles of construction of 

 the Animal Kingdom. 



The Undifferentiated Cell. — From this point of view the 

 primitive living organism is assumed to be an undifferentiated 

 cell, having no tissues, no organs, no permanently specialized 

 functions. If it moves, the motion is spontaneous, irregular, 

 temporary motion ; if it takes food, it is by attaching the 

 food to itself; and in a sense such a protozoal cell is all 

 mouth, all stomach, all everything necessary to living, but 

 nothing ])articular in any part of itself is permanent!}' different 

 from any other part: it is an undifferentiated organism. 

 The amoeba comes nearest to fulfilling these homogeneous 

 conditions, but even there appear the nucleus and the con- 

 tractile vacuoles, which are differentiated, and perform some, 

 though not well understood, special functions. 



cv 



Fig. s\.^Amoeba proteus (after Griiber), greatly enlarged. ci/ = contractile vacuole, n = 

 nucleus, /s = pseudopodium. 



In the simplest form of the metazoal cell very considerable 

 complexity is found at the earliest stage in which the cell is 

 observed. The steps by which the cell reaches the organic 

 structure which is characteristic of any of the mctazoa when 

 adult is explained in works on embryology and animal mor- 

 phology.* 



When we look at the progress more rapidly, and note the 

 steps of progress in function rather than in structural mor- 



*See McMurrich, "Text-book of Invertebrate Morphology," chapter ix., 

 Subkingdom Metazoa. 



