300 PHYSIOLOGICAL DEVELOPMENT. 



tissues answering to this primary contrast of conditions, is 

 no less conspicuous in aggregates of the second order. The 

 feebly-integrated units of a Sponge, with individualities so 

 little merged in that of the whole they form that most of 

 them still retain their separate activities, nevertheless show 

 us, in the unlikeness that arises between the outermost layer 

 and the contained mass, the effect of converse with unlike 

 conditions. This outermost layer is composed of units some- 

 what flattened and united into a continuous membrane a 

 kind of rudimentary skin. 



Secondary aggregates in which the lives of the units are 

 more subordinate to the life of the whole, carry this dis- 

 tinction further. The leading physiological trait of every 

 ccelenterate animal is the divisibility of its substance into 

 endoderm and ectoderm the part next the food and the 

 part next the environment. Fig. 147 ( 201), representing a 

 portion of the body-wall of a Hydra seen in section, gives 

 some idea of this fundamental differentiation. The creature 

 consists of a simple sac, the cavity of which is in communi- 

 cation with the surrounding water; and hence the unlike- 

 ness between the outer and inner layers has not become 

 great. The essential contrast is that between the differen- 

 tiated parts of what was originally the same part a uniform 

 membrane composed of juxtaposed cells. 



For here, indeed, we are shown unmistakably how the 

 primary contrast of structures follows upon the primary con- 

 trast of conditions. The ordinary form from which low 

 types of the Metazoa set out, is a hollow sphere formed of 

 cells packed side by side a blastula, as it is called : all these 

 cells being similarly exposed to the environment. The 

 blastula presently changes into what is called a gastrula a 

 form resulting from the introversion of one of the sides of 

 the blastula. If there be taken a small ball of vulcanized 

 india-rubber, say an inch or more in diameter, and having a 

 hole in it through which the air may escape, and if one side 

 of it be thrust inwards so as to produce a cup, and if the 



