I.] THE YELLOW YOLK. 7 



white yolk is generally known as the "nucleus of 

 Pander." 



Concentric to the outer enveloping layer of white 

 yolk there are within the yolk other inner layers of the 

 same substance, which cause sections of the hardened 

 yolk to appear to be composed of alternate concentric 

 thicker laminae of darker (yellow) yolk, and thinner 

 laminae of lighter (white) yolk (Fig. 1, w y ?/.). 



The microscopical characters of the white yolk 

 elements are very different from those of the yellow 

 yolk. It is composed of vesicles (Fig. 2, J?.) for the most 

 part smaller than those of the yellow yolk (4yi6 75^), 

 with a highly refractive body, often as small as 1/-&, in 

 the interior of each ; and also of larger spheres, each of 

 which contains a number of spherules, similar to the 

 smaller spheres. 



Another feature of the white yolk, according to His, 

 is that in the region of the blastoderm it contains 

 numerous large vacuoles filled with fluid; they are 

 sufficiently large to be seen with the naked eye, but do 

 not seem to be present in the ripe ovarian ovum. 



It is now necessary to return to the blastoderm. In 

 this, as we have already said, the naked eye can distin- 

 guish an opaque white rim surrounding a more trans- 

 parent central area, in the middle of which again is a 

 white spot of variable appearance. In an unfecundated 

 cicatricula the white disc is simply marked with a 

 number of irregular clear spaces, there being no proper 

 division into a transparent centre and an opaque rim. 



The opaque rim is the commencement of what we 

 shall henceforward speak of as the area opaca; the 

 central transparent portion is in the same way the 



