284 PHYSIOLOGICAL DEVELOPMENT. 



deepened until it becomes jar-shaped ; and finally, b}' narrow- 

 ing its neck, vase-shaped. And conceive the stretched film 

 to continue everywhere covering the surface during these 

 changes of form. What will finally be the relations of the 

 parts to one another ? The caoutchouc will line the inside of 

 the vase as well as coat its outside. The vase will consist of 

 a stratum of the clay included between the two India-rubber 

 surfaces. We shall have a distribution of layers answering 

 completely to the distribution of tissues in the Hydra. Now 

 if we imagine that this artificial layer which has covered the 

 clay during its changes of form, is produced by transforma- 

 tion of the clay, we shall see that when the mass is changed 

 into the vase-shape, the surfaces that have becom.e outer and 

 inner will develop in opposite directions from the substance 

 lying between them ; just as do the Hydra's ectoderm and 

 endoderm. And if, once more, we conceive these outer and 

 inner surfaces so resulting, to be affected by conditions some- 

 what unlike — the one by matters placed in the jar, and the 

 other by the medium surrounding the jar — we shall have, in 

 the slight difference produced between them, a difference 

 corresponding to that between the surfaces of the Hydra'fs 

 stomach and skin. 



Besides being able thus to understand how an aggregate 

 of Amceha-iovm units, originally coated by a single layer, 

 njay pass into an aggregate composed of a double layer ; we 

 may also understand under what influences the transition 

 takes place. Jf the habit wdiich some of the primary aggre- 

 gates have, of wrapping themselves round masses of nutri- 

 ment, is followed by a secondary aggregate, there will 

 naturally arise just that re-differentiation which the Hydra 

 shows us. 



§ 289. These duplicated surfaces which we see in every simple 

 conlenterate animal, are re- duplicated in all animals of higlier 

 classes — the more developed Cceleuieraia themselves showino 

 UE the transition. " Compared with the Hydroid Polypes,' 



