82 TEXT-BOOK OF EMBRYOLOGY. 





only active part; in the gastrula the two primary germ layers are simply 

 spread out on the surface of the yolk mass; the appearance of the mesoderm 

 does not alter the general conditions. Finally, however, in the latter part 

 of the incubation period, the germ layers grow over the entire surface of 

 the yolk like membranes; but this does not imply that the yolk is enclosed 

 within the embryonic body. 



The body of the embryo arises from a relatively small portion of the 

 blastoderm, and its position is indicated by the primitive streak and head 

 process which lie in its long axis. As they develop, the neural tube and 

 mesodermal somites produce a thickening of the blastoderm which becomes 

 slightly elevated like a rounded ridge above the general level of the surface 

 (Fig. 52). Just in front of the cephalic end of the neural tube a transverse 

 crescentic groove, the head fold, appears and sharply delimits the cephalic 

 end of the body. The rapidly growing brain then projects over the groove, 

 giving the appearance that the germ layers at the bottom of the groove are 

 being tucked beneath the head-end of the embryo. The horns of the cres- 

 centic head fold extend caudally along the sides of the thickening produced 

 by the neural tube and somites, thus delimiting the body region laterally. 

 These lateral grooves become deeper from before backward and eventually 

 reach the level of the primitive streak. Finally a transverse fold, the tail 

 fold, appears caudal to the primitive streak and becomes continuous on 

 each side with the lateral fold. The whole embryonic body thus becomes 

 surrounded and delimited by a gutter. 



The gutter or groove becomes deeper as the embryo continues to grow; 

 the head-end of the body projects farther over the head fold and the tail- 

 end farther over the tail fold. The effect is much as if the body was being 

 constricted or pinched off from the remainder of the blastoderm. The 

 remainder of the blastoderm, composed of the extraembryonic portion of 

 the germ layers, engages in the development of the yolk sac and certain 

 other appendages which are useful during the period of incubation. The 

 yolk sac and not the embryonic body contains the yolk that was present in 

 the ovum. The yolk sac is an appendage of the gut, it is true, but the yolk 

 substance is carried to the embryo by the blood circulating through the 

 vitelline vessels. 



References for Further Study. 



BLOUNT, MARY: The early Development of the Pigeon's Egg with especial Reference 

 to the Supernumerary Sperm Nuclei, the Periblast and the Germ Wall. Biological 

 Biilletin, Vol. 13, 1917. 



DUVAL, M.: Atlas d'Embryologie. Paris, 1889. 



HARPER, E. H.: The Fertilization and early Development of the Pigeon's Egg. Am. 

 Journal of Anat., Vol. 3, 1904. 



