EMBRYOLOGY 235 



recapitulation theory is plainly correct is in the development of gill 

 pouches in all of the vertebrates. Gills are never developed in the rep- 

 tiles, birds, and mammals; but gill pouches are formed, and these may 

 actually open temporarily to the outside as gill clefts, between which are 

 the gill bars upon which gills are developed in fishes and amphi})ia (frogs, 

 toads, etc.). The production of gill pouches and bars in the higher ver- 

 tebrates as well as in the lower, besides indicating a common ancestry of 

 all these animals, points with much certainty to the conclusion that the 

 ancestor was a fish-like animal — fish-like at least to the extent of being 

 aquatic and respiring by means of gills. 



MECHANISM OF DEVELOPMENT 



The practical identity of the steps of development in all members of 

 the same species of animal creates the problem of the mechanism of this 

 development. What insures that, at a given stage in cleavage, the vege- 

 tative half of an embryo shall be invaginated within the animal half? 

 What causes the neural folds to arise always in the same way, and at the 

 same relative time in development? Why does an eye always begin as 

 an evagination at the side of the brain? Why does the vesicle of the inner 

 ear originate behind the eye, instead of in front of it, or in some other 

 location? What causes the protrusion of the endoderm just behind the 

 early stomach, and the subsequent branching of this outgrowth to form 

 a liver? 



Explanations. — Most of these changes can be explained as due to 

 inequalities of growth in cells and layers of cells. A layer of ectoderm 

 must bend or fold if at some point one surface of it grows more rapidly 

 than the other. Increase in the size of one cell displaces the cells adjoin- 

 ing it. Actual migration of cells may also be appealed to in the explana- 

 tion of certain developmental changes. 



These explanations are not, however, very far-reaching, for they omit 

 the factors that cause the migration or inequalities in growth of cells. 

 Physiological processes are at the basis of all embryonic development, 

 but the nature of these processes is obscure. It has been suggested that 

 cells react to stimuli, just as organisms react to stimuli. Many animals 

 appear to act in purely mechanical ways in response to changes in their 

 environment, and it is not unlikely that cells of embryos have a similar 

 behavior. The stimuli to which they respond in their growth may be 

 chemical substances diffusing from the cells around them. Or mere 

 pressure of neighboring cells may act as a stimulus to specific kinds of 

 development. It cannot be too strongly emphasized, however, that 

 a statement of the cause of development must be largely conjectural. 



Independence of the Germ Cells. — A knowledge of the embryonic 

 development of animals leads to a conception of the relation of the germ 



