32 THE ESSENTIAL CHARACTERISTICS 



vesicle, the so-called germinal vesicle, than a clear spot in the 

 midst of the yolk granules. Were it not for the subsequent 

 development of these, Balbiani could hardly have determined 

 their true nature. 



If, therefore, such simple cells are really eggs, we must of neces- 

 sity form a new diagnosis of the characteristics of the theoretical 

 or typical egg ; and that I have, for one, been long inclined to 

 do. For the last five or six years I have felt that my studies 

 were leading me to look upon the relations of the various regions 

 of the egg as those of degrees, which shade off, the one into 

 the other. 



Our ideas of an egg have been heretofore based upon the 

 structure of the eggs of the higher animals, instead of upon what 



is common to all eggs. For instance, some 

 eggs of the higher animals (fig. 12) have 

 a germinal (or Purkinjean) vesicle ; within 

 that a germinal spot (or Wagnerian vesicle), 

 and within the last a nucleolus ; all equally 

 and sharply defined, like so many hollow 

 Fig. i2.~ ' concentric spheres, each and every one hav- 

 ing its peculiar, characteristic feature. Under this guise one 



would hardly suspect the true relations of 

 these parts to each other. But these I will 

 explain presently. In the eggs of some 

 other animals (fig. 13) there are but two of 

 these concentric vesicles, (namely, the Pur- 

 kinjean p, and the Wagnerian w,) the in- 

 nermost one being absent. Again there 

 Fi e- 13 - are those which have but one vesicle: for 



Fig. 12. The egg of the So w. 166diam. Natural size -^^ of an inch. The 

 next to the outer circle forms the boundary of the yolk. The small circle at the 

 upper side is the germinal or Purkinjean vesicle ; the oval spot is the germinal 

 dot or Wagnerian vesicle ; and the central spot is the nucleolus. From 

 Thompson. 



Fig. 13. Egg of the Rabbit. 166 diam. z, the " zona pellucida" or yolk 

 envelope; y, the yolk; p, the Purkinjean (germinal) vesicle; w, the Wagnerian 

 vesicle or nucleolus. From Coste. 



