ORIGIN OF CELLS. 



23 



large number of cells has resulted from the segmentation of the parent 

 cell. 



These cells arrange themselves into an upper la}^er, with their long 



Fig. 12.— Surface View of the Germinal Disk of a Hen's Egg during 

 the Later Stages of Segmentation. {Foster and Balfour.) 



At C, in the centre of the disk, the segmentation masses are very small and numerous ; at B, nearer 

 the edge, they are larger and fewer ; while those at the extreme margin, A, are largest and fewest of all. 

 It will be noticed that the radiating furrows marking off the segments, A, do not reach to the extreme 

 margin, E, of the disk. 



The drawing is complete in one quadrant only. It will, of course, be understood that the whole circle 

 should be filled up in a precisely similar manner. 



axes vertical, their nuclei become distinct, while the lower cells remain 

 large and granular and irregular^' placed, forming in this waj- the unin- 

 cubated blastoderm alread}' described. (See Fig. 10.) 



Fig. 13.— Section of the Germinal Disk of a Fowl's Egg during the 

 Later Stages of Segmentation. {Foster and Balfour.) 



This section, which represents rather more than half the breadth of the blastoderm (the middle line 

 being shown at C), shows that the upper and central parts of the disk segment faster than those below and 

 toward the periphery. At the periphery the segments arc still very large. One of the larger segments is 

 shown at A. In the majority of segments a nucleus can be seen ; and it seems probable that a nucleus is 

 present in them all. Most of the segments are filled with highly refracting spherules, but these are more 

 numerous in some cells (especially the larger cells near the /elk) than in others. In the central part of 

 the blastoderm the upper cells have commenced to form a distinct layer. No segmentate on -cavity is present. 



A, large peripheral cell; B, larger cells of the lower parts of the blastoderm; C, middle line of blasto- 

 derm j E, edge of the blastoderm adjoining the white yelk ; W, white yelk. 



As a result of incubation a third layer of cells makes its appearance 

 between the two layers of the blastoderm just described, forming an 

 upper, a middle, and a lower layer, or the epiblast, the mesoblast, and the 



