32 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



The development of tissue starts in the earliest stages of develop- 

 ment of the egg. 



We have seen already that, as a result of fecundation, the egg divides 

 into a number of minute segmentation spheres which at first form a 

 solid, mulberry -like mass, and we have now to consider the changes 

 occurring in the mass of simple, undifferentiated cells which result in the 

 development of the tissues and organs of the completed organism. 



In their early stages, as in the amoeba, each of these cells possesses 

 the powers of development, reproduction, growth, assimilation, respira- 

 tion, and contractility. As the organism passes to a higher stage we 

 find that many of these cells lose these general properties possessed in 

 their entirety by undifferentiated protoplasm, while certain of them are 

 put aside to carry on specific individual functions. Thus, in the young 

 embryo, as in the amoeba, all the cells possess the power of contractilit}' ; 

 as the organism develops, this property becomes restricted to cells form- 

 ing constituents of certain tissues, the contractile tissues, or the muscular 

 system. The amoeba, which we have already seen maj^ be regarded as 

 representing one of the units of which the higher organisms are built up, 

 possesses the power of irritability and automatism. In higher forms 

 this property of undifferentiated protoplasm is restricted to a single 

 tissue, the nervous system. The amoeba has no part specialized for the 

 various processes of nutrition; any part of its substance may take in 

 nutritive matter, may digest out the portions capable of supplying its 

 nutritive needs, may remove the undigestible residue : any portion of 

 the amoeba is capable of carrying on the metabolic processes by which 

 the matter absorbed as food is converted into protoplasm like itself, and 

 any portion is capable of absorbing the gases necessary for these com- 

 plex chemical processes and of getting rid of the effete products of its 

 nutritive processes. In the higher organisms certain cells are set aside 

 to form the organs concerned in the prehension of food ; others have for 

 their sole function the secretion of solvent juices which will digest out 

 the nutritive matters of the food ; others are the carriers of the matters 

 absorbed to remote corners of the organisms ; and the sole business of 

 certain other cells is to get rid of the useless matter and the products 

 of the waste of the economy. In the amoeba any portion may divide off 

 from the parent stock and so originate a new individual; in the higher 

 animals certain tissues or collections of cells have for their sole office 

 the reproduction of cells which shall constitute the starting point of a 

 new organism. In the amoeba, therefore, specialization of function has 

 not commenced ; each minute particle of the protoplasm which constitutes 

 it is capable of carrying on all the vital functions. In the higher organ- 

 isms, however, the elementary organisms of which they are built up are 

 so arranged that there may be a division of labor. These collections of 



