ORIGIN OF CELLS. 



21 



the blastoderm when the egg is first laid, and then the changes which 

 have preceded it. 



The blastoderm of an unincubated fertilized egg may be recognized 

 by the naked eye, when viewed from above, to consist of two parts: an 

 opaque, white circumference, the area opaca, and a central transparent 

 portion, the area pellucida. In the unfertilized egg these divisions are 

 not marked. They are due simply to the way the blastoderm, which is 

 itself entirely transparent, rests on the white yelk. The opaque, circular 

 ring is where the blastoderm is directly in contact with the white yelk, 

 while the central clear portion is due to the fact that the blastoderm is 

 separated from the yelk by a layer of liquid. The white spot often seen 

 in the centre of the blastoderm is the central column of white yelk 

 shining through the transparent membrane (Nucleus of Pander}. 



When the blastoderm is hardened and cut into vertical sections, it 

 is found to be composed of two layers of cells : the upper, small, nucle- 

 ated, cylindrical cells adhering closely together in a single layer and 



Fig. 10.— Section of as Uxlncubated Blastoderm of Chick. {Alein.) 

 A, cells forming the ectoderm ; B, cells forming the endoderm ; C, large, formative cells : F, segmen- 

 tation cavity. 



resting on the white yelk; the lower, an irregular net-work of larger cells 

 which are not nucleated, apparently, but which contain numerous highly 

 refractive granules. These are probably identical with the white-yelk 

 spheres already referred to, and are spoken of as formative cells. 



The processes which in the hen's body result in the formation of 

 such an egg are about as follow : 



In the capsule of the ovary the yelk alone constitutes the egg. It 

 then, just before bursting its capsule, consists of a minute, yellowish, 

 ellipsoidal, cellular body, with a delicate membrane, the vitelline mem- 

 brane, immediately below which in a granular cell-contents, the yelk, lies 

 a lenticular mass of protoplasm, the germinal disk; within this again is 

 a nucleus, the germinal vesicle, containing a nucleolus, or germinal spot. 



When the ovum becomes mature the ovarian capsule bursts, and 

 the ovum (representing the yelk of the egg as laid) escapes into the 

 oviduct, undergoes impregnation by the spermatozoa found in the upper 

 portion of the oviduct, and has deposited around it the accessory 



