44 ZOOLOGY. 



mother's body in incubation. For the details of further 

 development the student must be referred to more extensive 

 texts, but it may be stated that the blastoderm comes to consist 

 of two layers of cells which have been likened to two watch 

 glasses so placed as to enclose a shallow cavity. The outer 



FIG. 204. 



FIG. 204. Diagram of a longitudinal section of the embryo of a fowl, without the 

 amnion and allantois. Ectodermal boundaries are in continuous lines, the entodermal 

 and mesodermal are in broken lines: the entodermal of short dashes, the mesodermal of 

 long, b, brain; b.w., body wall: c.c., central canal of spinal cord; co., coelom; g, gut; 

 g.w., wall of gut: s.c., spinal cord; y.s., yolk sac. 



Questions on the figure. What is the relation of the yolk sac to the 

 digestive cavity? Which of the embryonic layers surrounds it? In what 

 way is the abundant yolk in the yolk sac brought into the circulation of the 

 embryo (see reference texts) ? 



layer is ectodermal and is continuous at the edge with the 

 inner, which is composed of larger cells incompletely separated 

 from the yolk beneath (Fig. n, C, 4). This inner layer gives 

 rise to both entoderm and mesoderm. The blastoderm contin- 

 ues to grow at the margins until the yolk is entirely enveloped 

 by a living membrane which is well supplied with blood vessels 



