34 



THE MODE OF DEVELOPMENT 



makes its first appearance in the egg-bearing organ, the ovary, 

 in the form of an indefinitely bounded globule, with a greater 



degree of transparency on one side than on 

 the other (fig. 15). This diversity in the 

 degree of refraction between the two sides is 

 owing to the difference in the nature of the 

 constituents of the globule ; on one side the 

 substance is pure albumen (alb.), whilst on 

 the other there is a certain amount of oleagi- 

 nous material (ol.) opposed to the albumen. Their boundaries, 

 however, are not definitely marked ; on the contrary, they insen- 

 sibly mingle with each other toward the centre of the egg, and 

 it is this indefiniteness that renders the aspect cloudy. Presently 

 the difference in the opposing features becomes stronger and 

 nib. more easily discerned ; the albuminous portion 



grows denser (fig. 16, alb.} and more decided in 

 character, whilst the oleaginous substance (ol.) 

 assumes a peculiar kind of refraction, totally dif- 

 ferent from that of the albumen ; and in the 

 Fig. 16. meanwhile the egg attains to a more clearly 

 defined outline, and grows larger. Soon, now, and while the 

 egg is yet very minute, the albuminous substance (fig. 17, alb.) 

 nib. rs. becomes more concentrated toward one side of 

 the egg, and assumes an appreciably definite 

 outline (p), but not as yet perfectly globular. At 

 the same time there is initiated within this con- 

 creted mass a similar condensation (w) at one 

 side. Coincident, usually, with this, the egg, 

 having continued to increase in size, becomes 

 very clearly defined in contour, and the superficial portion 

 develops into a more or less densely accreted envelope, which 

 bears the name of viteltine sac (vs.). The aim of all these pro- 



Figs. 15, 16, 17, 18. Theoretical eggs, representing the process of develop- 

 ment from the inception to completion, alb. the albuminous pole ; ol. the oleag- 

 inous pole; vs. the vitelline or yolk envelope; p, the Purkinjean (germinal) 

 vesicle; 10, the Wagnerian vesicle, or germinal dot. Original. 



