III.] THE HYPOBLAST. 51 



We must now go back, and say a few words about the 

 changes which the cells of the various layers undergo from 

 the 12 20 hours. 



FIG. 12. 



f/i, 



TRANSVERSE SECTION OF A BLASTODERM INCUBATED FOR 18 HOURS. 



The section passes through the medullary groove me., at some distance 

 behind its extreme front, and shews some of the chief points in which it differs 

 from the primitive groove. 



The chief of these are, (i) the presence underrieath it of the notoohord ch. 9 

 (2) the absence of any apparent adhesion between the epiblast and the meso- 

 blast, (3) the thickening of the mesoblast underrieath the medullary folds, mf. 



A. epiblast. B. mesoblast. C. hypoblast. 



m. c. medullary groove. nt< /. medullary fold. ch. notochord : the small group 

 of mesoblast cells separated by a narrow gap from the thicker mass of 

 mesoblast on either side. 



It is to be noticed that the cells of the hypoblast become more columnar as 

 they approach the edge of the pellucid area, and finally pass, without any strong 

 line of demarcation, into the white-yolk spheres. 



Only one half of the section is represented if completed the section would 

 be symmetrical about the line passing through the centre of the medullary 

 canal, me. , 



The hypoblast (Fig. 12, C) continues to be only one cell 

 deep ; the cells being, during the whole of this period, flatter 

 in the centre, and larger and more irregular towards the peri- 

 phery of the blastoderm. At about the 12th hour they are 

 very irregular in size ; shewing very great variations over 

 a very small space. This probably implies that they are 

 rapidly undergoing division. Later, however (about the 

 18th hour), they are fairly uniform over particular regions, 

 though they vary considerably in size at different parts of 

 the pellucid area. In no case does the hypoblast extend 

 beyond the edge of the pellucid area. 



The hypoblast cells along the central axis of the pellucid area, and for some 

 little distance on each side, are smaller than elsewhere over the blastoderm. 

 Over a small district just outside the embryo, and at about one-third of the 

 way from the posterior extremity of the blastoderm, they are, from the i8th 231x1 

 hour, considerably larger than anywhere else. The remaining hypobiast cells 



42 



