36 PHYSIOLOGY 



chloroplasts of vegetable cells are the most conspicuous. Certain cell 

 organs may fall into either class. Thus, the contractile vacuoles are 

 sometimes derived by the division of the pre-existing vacuoles in a 

 previous generation, at other times are certainly formed out of the 

 common cytoplasm. The centrosome, a small particle generally 

 situated in the cytoplasm, which plays an important part in cell 

 division, is generally derived by the division of a pre-existing centro- 

 some, but under certain conditions and in some organisms can be 

 developed in situ in the cytoplasm itself. 



The possibility of histological differentiation and of the adaptation 

 of structure to definite functions becomes much more pronounced 

 as we pass from the unicellular to the multicellular organisms or 

 metazoa. The lowest of the metazoa, such as the sponges, consist of 

 little more than an aggregation or colony of cells. All the cells are 

 still bathed with the outer fluid, and any differentiation of structure 

 or function seems to be entirely conditioned by the position of the cell. 

 In the ccelenterata the differentiation is already much more marked. 

 The hydra, one of the simplest of the group, consists of a sac formed 

 of two layers of cells and attached by a stalk to some firm basis. 

 Round the mouth of the sac is a circle of tentacles. The inner layer, 

 or hypoblast, represents the digestive and assimilatory layer, while 

 the epiblast, or outer layer, is modified for the purposes of protection, of 

 reception of stimuli, and of motor reaction. In the jelly-fish the differen- 

 tiation of the outer layers leads to the formation of the first trace of a 

 nervous system, i.e. a system fitted especially for the reception of stimuli 

 and for their transmission to the reactive tissues, namely, the muscles. 



In all these classes of animals the external medium of every cell 

 forming the organism is the sea- water or other medium in which they 

 live. This can penetrate through the interstices between the cells, 

 and every cell is therefore exposed to all the possible variations which 

 may occur in the composition of the surrounding medium. A great 

 step in evolution was accomplished with the formation of the ccelomata, 

 the class to which all the higher animals belong. In these, by the forma- 

 tion of a body cavity containing fluid, an internal medium is provided 

 for all the working cells of the body. The composition of this internal 

 medium is maintained constant by the activity of the cells in contact 

 with it, and the stress of sudden changes in the chemical composition of 

 the surrounding medium is borne entirely by the outer protective layer 

 of epiblast cells. These are rendered more or less impermeable by the 

 secretion on their surfaces of a cuticular layer, and only such of 

 the constituents of the surrounding medium are allowed to enter the 

 organism as can be utilised by it for building up its living protoplasm. 

 Out of the coelom is later on formed a circulatory system which, by the 

 circulation of the coelomic fluid or of blood throughout the whole 



