ANATOMY OF THE EGG. 743 



(2109.) The anatomy of the egg prior to the commencement of incu- 

 bation is therefore sufficiently simple. Immediately beneath the shell 

 is the membrana putaminis-, which, however, we must here remark, 

 consists of two layers ; and at the larger end of the egg these layers 

 separate, leaving a space (fig. 370, a, b), called the vesicula aeris ; we 

 may further notice that the chamber so formed is filled with air con- 

 taining an unusual proportion of oxygen, destined to serve for the respi- 

 ration of the future embryo. Enclosed in the membrana putaminis the 

 student next finds the albumen and chalazas (fig. 371, c) ; and lastly, 

 the yelk, enclosed in its proper membrane (fig. 370, c), the membrana 

 vitelli. 



(2110.) We must dwell a little more at length, however, upon the 

 composition of the yelk. The cicatricula (fig. 370, g} is made up of a 



Fig. 370. 



Anatomy of the egg. 



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. 



(2111.) Immediately over the blastoderm the membrana vitelli is 

 slightly thickened (fig. 370, h) ; and beneath it is a canal (e), which 

 leads to a chamber (d) placed in the centre of the yelk ; this cavity is 

 filled with a whitish granular substance. 



(2112.) 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 development of the embryo 

 is accomplished in the following manner. 



(2113.) No sooner has incubation* commenced, than the blastoderm 



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

 und Reflexion. 4to. 1837. 



