52 THE FIRST DAY. [CHAP. 



are intermediate in size between these very large cells and the smaller cells 

 in the centre. During the whole of this period the hypoblast cells continue 

 to be granular and filled with highly refractive spherules, exhibiting in this 

 respect a marked contrast to their appearance at a later time. 



Their mode of increase is partly by division, but the layer grows chiefly 

 in a manner which is very different and somewhat remarkable. Before the 

 1 2th hour the hypoblast at, its margin ended abruptly against the white-yolk 

 cells; but after that hour its relation to the white yo,k becomes altered. As 

 they approach the white yolk the cells of the hypoblast become more and 

 more filled with white-yolk spherules, and at the extreme edge of the pellucid 

 area it is very difficult to say where the white yolk ends, and where the 

 hypoblast begins. This is somewhat diagrammatically shewn in Fig. 12. The 

 white-yolk spheres near the edge of the pellucid area have generally acquired 

 nuclei, though it is frequently difficult to see them owing to the numerous 

 highly refractive spherules which the spheres contain. The nearer they are 

 to the edge of the pellucid area the fewer spherules they contain, and at the 

 very edge it is almost impossible to say whether they ought to be called white- 

 yolk spheres or hypoblast cells. The chief increase of the hypoblast therefore 

 seems to take place through the conversion, cell for cell, of the white yolk 

 into the hypoblast. 



During this period the mesoblast (Fig. 12, B) cells do 

 not undergo any marked change. The layer itself enlarges 

 to a certain extent through the multiplication of cells by 

 the division of old ones; but the chief increase in bulk is 

 probably due to the formative cells, which are continually 

 passing round from the bottom of the segmentation-cavity 

 to the mesoblast, and there become converted, in the way 

 described (§ 3) above, into mesoblast cells. 



These formative cells are more numerous at the bottom of the segmentation- 

 cavity at the 18th hour than they w^ere at the first hour. This accession to 

 their number is probably due to fresh ones being formed from the floor of white 

 yolk. They appear to grow in size by absorbing the white-yolk spherules, 

 with which indeed they are completely filled. 



The epiblast cells (Fig. 12, A) probably increase entirely 

 by division, and seem to derive their nourishment from the 

 white yolk on which the peripheral cells rest, and perhaps 

 also from the albuminous fluid which fills the segmentation- 

 cavity and occupies all the interstices between the cells of 

 the various layers. 



The cells near the edge of the opaque area are the largest and flattest 

 of the epiblast cells; those in the middle of the pellucid area are smaller than 

 those at its edge. 



Outside the blastoderm there are to be seen on the surface of the yolk 

 alternating transparent and opaque white rings. These are known as the 

 halones, and frequently appear at the commencement of incubation. It is 

 stated by His that they are to be explained by the white-yolk spheres under- 

 going changes of two kinds. In the one case the spherules they contain are 



