16 THE HEN'S EGG. [CHAP. 



appear to be composed of alternate concentric thicker laminae 

 of darker (yellow) yolk, and thinner laminae of lighter (white) 

 yolk (Fig. 1, w, y.). 



8. The microscopical characters of the white yolk are 

 very different from those of the yellow yolk. It is composed 

 of spheres (Fig. 2, S.) for the most part smaller than those of 

 the yellow yolk (4/x 75/z,), with a highly refractive nucleus- 

 like body often as small as l/^ in the interior of each; and 

 also of larger spheres, each of which contains a number of 

 spherules, similar to the smaller spheres ; these latter appear- 

 ing to have passed into the larger spheres, by a process of 

 inclusion. 



There has been a considerable amount of controversy as to whether these 

 elements possess a membrane ; there is little doubt however that there is no 

 membrane present. 



It has also been disputed as to whether they should be considered as true 

 cells or not. If by definition a cell must contain a nucleus, they can hardly 

 be considered as such, since the characters of the highly refractive bodies con- 

 tained in them have nothing in common with nuclei. We shall give later on 

 reasons for thinking that they may however, as a result of incubation, become 

 veritable cells. 



Another feature of the white yolk, according to His, is 

 that in the region of the blastoderm it contains numerous 

 large vacuoles rilled 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. 



9. It is now necessary to return to the blastoderm. In 

 this, as we have already said, the naked eye can distinguish 

 an opaque white rim surrounding a more transparent 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 trans- 

 parent portion is in the same way the beginning of the area 

 pellucida. At this stage the distinction between these two 

 areas depends entirely on the disposition of the white yolk 

 beneath them, for the blastoderm when lifted up from the 

 white yolk on which it rests appears uniform throughout. 

 In the part corresponding to the area opaca the blastoderm 

 rests immediately on the white yolk, which here forms a 



