THE EARLY DEVELOPMENT OF THE CHICK 245 



distinguished from the marginal cells, which remain incomplete 

 both below and distally, retaining their connection with the 

 periblast (Fig. 90, B}. A definite though narrow space appears, 

 accompanying the horizontal cleavage, which separates the 

 superficial cellular elements from the underlying undivided 



mp 



FIG. 90. Vertical sections through the chick blastoderm during the process 

 of cleavage. After Patterson. A. Section through the two-cell stage. B. 

 Median section through the thirty-two cell stage. C. Part of a longitudinal 

 section through the sixty-four cell stage, b, Blastoccel or segmentation cavity; 

 c, central cells; i, inner cell cut off by horizontal cleavage plane; I, neck of latebra; 

 m, marginal cell; mp, marginal periblast; n, nucleus; p, first cleavage; v, vitelline 

 membrane. 



protoplasm. This space is the beginning of the segmentation 

 cavity or blastoccel; the protoplasm beneath it is termed the 

 central periblast. The original periblast region is now distin- 

 guished as marginal periblast. The two periblastic regions 



