OF THE TYPICAL EGG. 35 



cesses becomes now rapidly apparent, for soon we find that 

 the albumen has clearly defined itself as a 

 separate mass, (fig. 18,) apart from the yolk 

 (o/.), and its superficies has condensed into a 

 well-marked envelope, which constitutes the 

 germinal vesicle (p), whilst the condensation, 

 going on within it at the last stage, has 

 resulted in the formation of a clearly estab- 

 lished agglomeration (w) with a distinct wall 

 around it, which is usually called, together with the contents, 

 the germinal dot, or sometimes the Wagnerian vesicle. Outside 

 of this field of operations, and antagonistic to it in character, 

 the yolk (<?/.) has its peculiarities, in physiognomy, refraction, 

 density, opacity, and color, according to the kind of animal in 

 which the egg develops; all tending to demonstrate that it is 

 under a different formative influence from that of the albumen, 

 at an opposite pole, we might say, from the latter.* 



We may, therefore, define an egg to be a globular accretion of 

 two kinds of fluids, albumen and oil, which are always situated at 

 opposite sides or poles ; but, at the point where they meet, there 

 are various degrees of separation, from the most sharply defined 

 line down to the most indefinite boundaries when they are more 

 or less mingled with each other. In the earliest stages of all 

 eggs, these two poles shade off into each other; but whilst some 

 do not develop above this condition, there are others which, as 

 we have seen, take on a higher form of specialization ; and in 

 this latter case the concentric vesicles of the egg would seem to 

 bear the character of special organs. 



The eggs of all birds, as I have reason to know from almost 

 innumerable observations, possess not only this high degree of 

 specialization in the albuminous region, but exhibit, to an extraor- 



* To such an extent is this antagonism, between the oleaginous and albuminous 

 components of the egg, carried out in some of the worms, e. g. Tsenia (fig. 

 44), Planaria (fig. 47), Prostomum (fig. 19), that the oleaginous portion, or 

 yolk, of the egg is developed in a different part of the body from where the 

 albuminous material originates : and it is not until a certain period that these 

 two are brought into proximity to each other to form a single egg. In this figure 



