200 



EM BR YOLOG Y 



rise are to be regarded as appendages which sooner or l^er 

 disappear or are ultimately cast off. 



The blastoderm, consisting originally of two layers, is soon 

 transformed into three fundamental germinal layers ; the upper, 

 middle, and lower layers, or epiblast, mesoblast, and hypoblast. Three 

 similar germinal layers are found in the embryos of all animals 

 with the exception of the lowest invertebi^ate forms, and their 

 history is one of the most important subjects of comparative 

 Embryology. 



The epiblast gives rise to the epidermis with its derivatives, to 

 the whole of the nervous system, and to the most important parts 

 of the sj^ecial sense - organs. The hypoblast furnishes the whole 

 secretory laj^er and epithelial lining of the alimentary canal and 

 its glands, with the exception of part of the mouth and anus, which 

 as invaginations of the outer layer are lined by the epiblast. Out 

 of the mesoblast the whole of the vascular, muscular, and skeletal 

 systems, and the connective tissue of all parts of the body are 

 developed, as well as the excretory organs and the generative glands. 

 The blastoderm gradually and uniformly expands as a thin 

 circular sheet over the yolk immediately beneath the vitelline 

 membrane. At last by the end of the seventh day of incubation, 

 the Avhole mass of the 3-elloAv yolk becomes enclosed in a bag 

 formed by the blastoderm. This bag is formed chiefly by the area 

 opaca, the mesoblast of which produces numerous blood-vessels and 

 becomes transformed into the area vasculosa. 



The embryo itself is formed by a folding-off of the central portion 

 of the area pellucida from the rest of the blastoderm ; a semilunar 

 groove or tucking-in of the blastoderm appearing at the head end of 

 the future embryo is spoken of as the " headfold." In an eggjDlaced 

 before us with its blunt end towards the right-hand side, the head- 



LONGITUDINAL AND VERTICAL SECTIONS THROUGH TrUXK OF AS EmERYO, E (shailed), 



ON THE Second, Fourth, and Sixth Days. 

 a.A.V. Anterior amniotic fold ; i^.A.¥. Posterior amniotic fold ; c, Plenro-peritoneal cavity ; 



y.s. Yolk-sac ; Al. Allantois. 



fold invariably looks away from us, and the longitudinal axis of 

 the future embryo stands at right angles to the long axis of the 



