18 THE hen's egg. [chap. 



the female pronucleus to constitute the nucleus of the 

 impregnated ovum. 



Segmentation. There follows upon the impregna- 

 tion a remarkable process known as the segmentation. 

 The process consists essentially in the division of the 

 impregnated ovum by a series of successive segmenta- 

 tions into a number of cells, of which the whole of the 

 cells of the future animal are the direct descendants. 

 In the majority of instances this process results in the 

 division of the whole ovum into cells ; but in cases of 

 ova where there is a large amount of food yolk, only 

 that part of the ovum in which the protoplasm is but 

 slightly loaded with food material, and which we have 

 already described as the germinal disc, becomes so 

 divided. The remainder of the ovum constitutes a 

 food reservoir for the use of the developing embryo 

 and is known as the food yolk. The segmentation in 

 such ova, of which that of the fowl is one of the 

 best known examples, is described as being partial or 

 meroblastic'. 



In order to understand the process of segmentation 

 in the fowl's ovum it must be borne in mind that the 

 germinal disc is not sharply separated from the re- 

 mainder of the ovum, but that the two graduate insen- 

 sibly into each other. 



The segmentation commences in the lower part of 

 the oviduct, shortly before the shell has begun to be 

 formed. 



Viewed from above, a furrow is seen to make its 



' For a fuller account of the relation between holoblastic and 

 meroMastio segmentation the reader is referred to the treatise on 

 Comparative Embryology by Balfour, Vol. i. chapter iii. 



