SEGMENTATION OF THE EGG. 233 



are also forming, by which the segments become completely 

 isolated from one another, and from the underlying yolk 

 (Fig. 104, za). These horizontal clefts, like the vertical ones, 

 appear first in the centre of the germinal disc, and do not reach 

 its margin until a later stage. 



In each segment, or cell as it may now be termed, a nucleus 

 is present from the first. The precise mode of origin of these 

 nuclei has not been determined with certainty, but the history 

 of the segmentation of the egg in Amphioxus, the frog, and other 



Fig. 102. Fig. 103. 



FlG. 102. — An early stage in the segmentation of the germinal disc of the 



Hen's Egg. (After Cos.te, and Duval.) x 10. 

 Fig. 103. — A later stage, in which the germinal disc has increased in size, and 



the segments have, by further division, become smaller and more 



numerous. (After Coste, and Duval), x 10. 



Both these figures are from eggs taken from the lower part of the 



oviduct of the hen. 



animals, leaves little doubt that the nuclei of all the cells are 

 derived, by division, from the single segmentation nucleus of the 

 fertilised egg. 



The result of the process of segmentation, up to the point 

 shown in Figs. 103 and 104, is the formation of a cap, occupying 

 the centre of the germinal disc, and consisting of a single layer 

 of nucleated cells : of these, the central ones, za, are small, and 

 completely isolated from their neighbours, and from the under- 

 lying yolk ; while the marginal ones, ZB, are larger, and are 

 only imperfectly marked off from the yolk, the horizontal clefts 

 having not yet appeared. 



The process of segmentation soon extends into the deeper 

 part of the germinal disc ; and by a further series of clefts, in 

 different planes, this deeper part of the disc becomes cut up into 



