AVKS BIRDS. 



621 



the vesicula aeris , 

 so formed is filled 

 with air containing 

 an unusual propor- 

 tion of oxygen, des- 

 tined to serve for the 

 respiration of the 

 future embryo. En- 

 closed in the mem- 

 brana putaminis the 

 student next finds 

 the albumen and 

 chalazas (Jig. 285, 

 c) ; and lastly, the 



we may further notice, that the chamber 



Fig. 284. 



the 



mem- 



yolk, enclosed in its proper membrane (Jig. 284, c), 

 brana vitelli. 



(698.) We must, however, dwell a little more at length upon 

 the composition of the yolk. The cicatricula (Jig. 284, g) is made 

 up of a thin membrane, which originally enclosed the vesicle of 

 Purkinje (/); but this latter, although introduced into the diagram 

 for the purpose of illustration, is in reality, as we have already seen, 

 no longer visible ; and we must now change the word cicatricula 

 for that of blastoderm, which may be presumed to consist of the 

 original cicatricula and the ruptured vesicle of Purkinje : it is from 

 this blastoderm, or germinal membrane, as it is sometimes called, 

 that the future being is developed. 



Immediately over the blastoderm the membrana vitelli is slightly 

 thickened (Jig. 284, F^.285. 



h) ; and beneath it 

 is a canal (e), which 

 leads to a chamber (d) 

 placed in the centre of 

 the yolk; this cavity 

 is filled with a whitish 

 granular substance. 



(699.) Such is the 

 composition of the 

 complete egg of a 

 Fowl ; and with the 

 exception of trifling circumstances, hereafter to be noticed, of 

 that of vertebrate animals in general. The develppement of the 

 embryo is accomplished in the following manner. 



