308 



.EMBRYOLOGY. 



SEC01SD PEEIOD OF THE DEVELOPMENT OF THE CHICK, TO 

 THE EVOLUTION OF THE SECOND CIECULATION. 



[ 487. THE second period in the history of the development 

 of the chick begins with the third day, in the course of which 

 the circulation in the vitelline vessels is completely established 

 (figs. 339 and 346), and embraces farther the changes that 

 take place during the fourth and fifth days, till the allantois 

 has appeared, the membrane of the shell has been attained, 

 and the second circulation is established ; the first, which had 

 reached its highest development at the end of the fourth day, 

 now beginning to suffer an arrest, and to decline in extent and 

 activity (figs. 341 and 345). In the course of this period the 

 embryo is completely detached from the germinal membrane, 

 and becomes enveloped in peripheral productions of the same 

 part. The third day is the most remarkable in the whole 



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Fig. 338. Ideal section of an embryo somewhat younger than that of 

 fig. 339. A, transverse section ; a, vitelline membrane ; b,b, laminae dorsales 

 et vertebrales ; b-, b~, laminae abdominales and transverse processes ; c,c, 

 lamina mucosa, which is seen bending round under the chorda dorsalis (e), 

 to form the intestinal canal ; d, d, lamina vasculosa ; /, /, peripheral por- 

 tion of the lamina serosa, proceeding to form the lateral involucra and the 

 amnion ; g, medulla spinalis. B, longitudinal section ; a, vitellary mem- 

 brane ; b, lamina serosa, and dorsum of the embryo ; b~, head of the 

 embryo ; c, c, lamina mucosa ; d, lamina vasculosa ; d 2 , heart ; d 3 , branchial 

 arteries ; c? 4 , aorta ; d 5 , artery of the blastoderma (arteria vitellina). 



