TEGUMENTARY ORGANS 



367 



the proper tegumentaiy system and a great part of the nervous, the 

 muscular, and the vascular systems ; but what I wish to direct attention 

 to at this moment, is the fact, that the first differentiation into integu- 

 ment and viscus proceeds from the setting up of two independent lines, 

 ■or rather planes of growth, in the germinal membranes. 



In the Hydra and Hydroid Polypes generally, we have the essence 

 of this embryonic state as a persistent condition. If, in fact, the body 

 or almost any organ of one of these animals be examined, it will be 

 found {see Memoir on the Structure of the Medusse, Phil. Trans. 

 1849) to be composed of two distinct membranes, an inner and an 

 ■outer [fig. 303. A). The junction between the two is distinctly marked 

 by a clear line, which would elsewhere be called a basement membrane 

 (a). External and internal to this, there is a layer of young tissue, 

 consisting of a homogeneous periplast with minute imbedded endoplasts 

 {" nuclei "). As we proceed towards 

 the free surface, we find that a 

 process of vacuolation and cellu- 

 lation takes place in the periplast, 

 until the coarsely cellular appear- 

 ance with which every one is 

 acquainted is produced. 



In the Hydra, then, we have 

 the whole thickness of the body 

 divided into two portions by a line, 

 on each side of which, inwards 

 and outwards, there is an increas- 

 ing histological metamorphosis or 



■differentiation. There is a median plane of no differentiation, as it 

 might be termed, external and internal to which, is a zone of 

 indifferent tisstce, while, still more remote again, is a zone of meta- 

 morphosed tissue. The absolute structure of the two layers thus 

 produced is very similar, ^ so much so, that, as is well known, either 

 may perform for a time the function of the other. The distinction 

 between the integument and the mucous membrane in a morphological 

 point of view, however, is as strongly marked as in the most complex 

 animal. The integument, in fact, grows from within outwards — it is 

 endogenous, its youngest portions being internal : the mucous 

 membrane, on the other hand, grows from without inwards — its 

 youngest portion is external, and it is, therefore, exogenous. 



We have here, I believe, the fundamental, and the only essential 

 ■distinction, between true integumentary or " epidermic " structures and 



' Though not, as it is commonly said, identical. 



Fig. 303. — A, hydra; d, outer membrane ; 



c, inner membrane. B, young mammal ; 



d, epidermis ; c, derma. 



