26 the hen's egg. [chap. I. 



We were thus led to the conclusion that a nucleus really existed in all. It is 

 of course quite possible that the clearer nucleated masses eventually come to 

 the surface and leave the more granular and opaque masses to form the lower 

 layer; but it is much more likely that they do not, and that the granular con- 

 dition of the cells of the lower layer of the fully formed blastoderm is on the 

 one hand the result of their being in immediate contact with the excessively 

 granular white yolk-cells, and on the other the cause of their nuclei not being 

 seen. We have a somewhat analogous case in the invisibility of the nucleus in 

 the early stages of the amphibian blood-corpuscle. 



At the time when the segmentation-spheres in the centre 

 are smaller than those at the periphery, and those above 

 are also smaller than those below, a few large spherical 

 masses begin to separate from the remainder (or to arise 

 by a continued process of segmentation from the bed of 

 white yolkj, and to rest directly on the white yolk, at the 

 bottom of the shallow segmentation-cavity. They contain 

 either numerous small nucleated spherules, or fine granules ; 

 the spherules precisely resembling the smaller spheres of 

 white yolk. These loose spherical masses are the formative 

 cells already spoken of. 



Thus the original germinal disc of the ovarian ovum, 

 its germinal vesicle having disappeared, becomes, by the 

 process of segmentation, converted into a blastoderm such as 

 is met with in the egg when laid, into an upper layer of 

 columnar nucleated cells, and into a lower layer of irregularly 

 disposed rounded masses which have not yet definitely ac- 

 quired the character of cells, accompanied by a few stray 

 " formative " cells lying loose in the segmentation-cavity. 



