Reproduction 297 



the epidermis of amphibians and the external gills (larval or perma- 

 nent) of some fishes and amphibians. 



Significance of Gastrula. An animal carries on perpetual inter- 

 change of materials, nutritive and respiratory, with its environment, 

 and must be able to react to external stimuli in a manner favoring 

 survival. Digestion and absorption of food are best effected at an 

 internal surface. Protective structures and reaction mechanisms must 

 be at, or in close relation to, an external surface. Respiration can be 

 provided for at either an external or an internal surface. 



The gastrula is a multicellular animal in its bare essentials. The 

 outer layer, ectoderm, gives rise to the essential outer part of the 

 adult skin, which produces so many important protective structures, 

 and to the whole nervous system, both peripheral and central. The 

 inner layer, endoderm, is nutritive. The cavity within it is the pri- 

 mary digestive cavity. It is significant that the wall of the archenteron 

 is derived from the vegetal hemisphere of the blastula. Thus, appro- 

 priately, the greater quantity of yolk comes to lie in the lining of the 

 embryonic digestive cavity. In the vertebrates the blastopore never 

 becomes mouth and rarely becomes anus. The future motor mecha- 

 nism, muscle, is derived indirectly from the gastrula layers. 



In view of the fact that all vital functions are carried on in an 

 organism so small as to be organized as a single cell, it is conceivable 

 that the minimum multicellular animal might be a gastrula-like thing 

 but with a wall only one cell thick. But, in accord with the principle 

 of specialization or "division of labor," two layers of cells, each layer 

 structurally specialized for its peculiar activities, are functionally more 

 efficient than one layer could be. 



The gastrula is strongly suggestive of the two-layer body plan of 

 a coelenterate. A simple coelenterate such as Hydra, two-layered 

 throughout, including even the tentacles, can be regarded as a some- 

 what elaborated gastrula, the Hydra "mouth" corresponding to the 

 blastopore (Fig. 285). Contractile fibrous processes are produced by 

 cells of both layers, forming a very thin sheet of tissue, essentially 

 muscular, between ectoderm and endoderm. The gastrula-like form of 

 the coelenterate, together with the fact that a gastrula stage, modified 

 in one way or another, occurs nearly universally in the development of 

 metazoan animals, gave rise to Haeckel's"gastraea" theory, which pro- 

 posed that gastrula-like animals (essentially coelenterates) must have 

 been the ancestors of all Metazoa. According to this theory, the occur- 

 rence of the gastrula form in the ontogeny of a vertebrate is a "repe- 

 tition" of the coelenterate stage in phylogeny. This may very well be 

 true, but it is not necessary to hold this view in order to account for 



