622 



AVES BIRDS. 



Fig. 286. 



No sooner has incubation* commenced, than the blastoderm be- 

 comes distinctly separate from the yolk and the membrana vitelli; 

 and, as it begins to spread, assumes the form of a central pellucid 

 spot, surrounded by a broad dark ring (Jig. 285, g, h) : it at the 

 same time becomes thickened and prominent, and is soon separable 

 into three layers; of these, the exterior (Jig. 286, c) is a serous 

 layer ; the internal, or that next the yolk (A), a mucous layer ; 

 and between the two is situated a vascular layer, B, in which vessels 

 soon become apparent. These three layers are of the utmost im- 

 portance ; as from the first mentioned, all the serous structures, 

 from the second all the mucous structures, and from the third 

 the entire vascular system of the embryo originate. 



(700.) Towards the close of the first day of incubation the 

 blastoderm has 

 already begun to 

 change its appear- 

 ance, and two 

 white filaments are 

 apparent in the 

 middle of the cen- 

 tral pellucid cir- 

 cle. Supposing a 

 longitudinal sec- 

 tion of it at this 

 period, the mem- 

 brana vitelli will 

 be found to have 

 become more pro- 

 minent where it passes over the germinal space (Jig. 286, 1, p). 

 The outer layer of the blastoderm (c) has become thickened at e 

 into the first rudiment of the dorsal portion of the embryo ; but the 

 mucous layer A, and the vascular layer B, have as yet undergone 

 little alteration. 



At the commencement of the second day (Jig. 286, 2), the an- 

 terior portion of the embryo is dilated, and bent down so as to 

 inflect the three membranes of the blastoderm at this point. 



At the conclusion of the second day this inflection is carried still 

 further; and from the vascular layer, a single pulsating cavity 

 (Jig. 286, 3, A), the punctum saliens, the first appearance of a 



* Dr. Karl Ernst v. Baer iiber Entwickelungsgeschichte der Thiere. Beobachtung 

 und Reflexion. 4to. 1837. 



