732 FORMATION OF THE EMBRYO. 



the egg of the frog. In the frog, the whole of the blastoderm serves for 

 the formation of the body of the embryo. In the fowl, only a portion 

 of it is immediately devoted to that object ; while the remainder extends 

 itself over the voluminous yolk, to be employed for the absorption of 

 nutritious material and its indirect transfer to the embryonic tissues. 



But even within the limits of the body of the embryo, similar folds 

 of the blastoderm become visible, and are the principal means of forma- 

 tion for its different organs. The earliest permanent appearances of 

 this kind are the longitudinal ridges which include between them the 

 "medullary groove" (Fig. 252, I.), and which afterward, by coalescing 

 with each other along the median line of the back, inclose the medul- 

 lary canal (Fig. 252, II.). That these ridges or " dorsal plates," as well 

 as the groove between them, are produced by the formation of folds, is 

 plain from the fact that the surface of the groove, while still open, is 

 continuous, over its undulating borders, with that of the neighboring 

 part of the blastoderm ; and that after its closure, its cavity is lined 

 with a layer of cells identical in form with those on the free surface of 

 the blastoderm above. It is also shown, by transverse sections of the 

 embryo (His, Foster and Balfour), that the folds in question pass 

 through the whole thickness of the outer blastodermic layer. Accord- 

 ing to Foster and Balfour, the medullary canal, in the fowl's egg, is 

 completely closed at the region of the head on the second day of incu- 

 bation ; after which the coalescence of its edges goes on progressively 

 from before backward. 



The closure of the abdomen in front, and the conversion of the inner 

 layer of the blastoderm into an intestinal canal, take place by a similar 

 production of lateral folds, approaching each other along the median 

 line. For, as the limits of the body of the embryo are marked off, on 

 each side, from the rest of the blastoderm by an inverted fold, when 

 this fold becomes deeper its borders are brought nearer to each other. 

 Thus the body of the embryo is at first spread out on the surface of the 

 vitellus, lying, as it were, upon the mucous membrane of its open alimen- 

 tary canal But as the folds which mark its lateral borders penetrate 

 more deeply below the surface (Fig. 252, IV.), the sides of the embryo 

 shut in between them a portion of this mucous membrane, and at last 

 completely inclose it in the abdominal cavity, in the same manner as the 

 dorsal folds inclose the medullary canal. 



The folds of the blastoderm, which thus determine the configuration 

 of the embryo, are the result of a special activity of growth in par- 

 ticular parts of the blastodermic layers. If the blastoderm were to 

 grow only at its edges, these would simply extend farther and farther 

 over the vitellus, the central portion remaining as before. Or if it were 

 to increase at a uniform rate in all its parts at the same time, its form 

 would not necessarily be subjected to any special alteration. This is 

 what really takes place during the production of the blastoderm itself. 

 The segmentation of the vitellus, and the organization of the cellular 

 layers, go on with a similar activity in all directions, extending uni- 



