200 EMBRYOLOGY 
rise are to be regarded as appendages which sooner or later 
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 invertebrate 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 special sense- organs. ‘The hypoblast furnishes the whole 
secretory layer 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 whole mass of the yellow 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 egg placed 
before us with its blunt end towards the right-hand side, the head- 
LONGITUDINAL AND VERTICAL SECTIONS THROUGH TRUNK OF AN Embryo, £ (shaded), 
ON THE SEconpD, FourtnH, AND S1xra Days. 
a.A.F. Anterior amniotie fold ; p.A.F. Posterior amniotic fold ; ce, Pleuro-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 
