302 PHYSIOLOGICAL DEVELOPMENT. 



might act upon the exterior. So that this introverted part 

 has a quasi-externality. It has not the same absolute in- 

 ternality as have those parts which never come in contact 

 with products of the outer world. Here we must briefly 

 recognize the distinction between these parts and the parts 

 thus far considered. 



Eeverting to our symbol, the india-rubber ball, it will be 

 seen that the introversion may be so complete that the cavity 

 is obliterated, with the result that the internal surfaces of 

 the outer and inner layers come in contact. This is the 

 state reached in the simplest ccelenterate animal, the Hydra: 

 there being in it nothing more than a thin structureless 

 lamella between the ectoderm and endoderm, as shown in 

 Fig. 147. This lamella represents all that there is of strictly 

 internal tissues. But the introversion, instead of bringing 

 the inner surfaces of the ball into contact, may be so far in- 

 complete as to leave a space, and in various creatures and 

 embryos of others, symbolized by this arrangement, this space 

 becomes occupied by a tissue formed from one or other or 

 both of the two primary tissues — the mesoblast or meso- 

 derm. This intermediate layer, sometimes, as in the Medusa, 

 growing into a mass of jelly serving as a fulcrum for the 

 creature's contractions, or, as in the Sponge, giving a passive 

 basis to the active tissues, becomes in higher animals the 

 layer out of which the structures that support the body and 

 move it about, as well as those that distribute prepared 

 nutriment, are developed. From it arise the bones, the 

 muscles, and the vascular system — the masses of differen- 

 tiated tissue which are truly internal and occupy what is 

 called the body-cavity or peri-visceral space. 



In the higher types of animals this space comes to be 

 partially occupied by a structure that may be described as a 

 cavity within a cavity — the ccelom. Most zoologists regard 

 this as arising by a re-introversion of the archenteron or 

 primary alimentary sac. It is easily to be perceived that 

 after the introversion which produces this digestive cavity, the 



