68 TEXT-BOOK OF EMBRYOLOGY. 



the second cleavage is again indicated by a slight furrow near the center of 

 the disk. The first four blastomeres are separated only near their apices, 

 the peripheral region remaining unaffected (Fig. 38, a). The third cleavage 

 has not been observed in the hen's egg. As a rule the fourth cleavage tends 

 to cut off the apices of the preceding blastomeres so that a central group of 

 small cells is surrounded by a peripheral group of large cells which are not 

 completely divided (Fig. 38, b and c). About this time some of central cells 

 also divide in the horizontal plane. In subsequent divisions the central 

 continue to divide more rapidly than the marginal cells, although among 

 the latter the intercellar boundaries are extending still farther toward the 

 dge of the disk (Fig. 38, d). As divisions succeed one another some of the 

 nuclei of the marginal cells migrate out into the yolk surrounding the disk, 

 the cytoplasm also encroaching upon the yolk and mingling with it. In 

 this manner the disk gradually becomes more extensive in all directions. 



FIG. 39. From a vertical section through the germ disk of a fresh-laid hen's egg. Duval, Hertni'g. 

 g.d., Upper layer of germ disk; s.c., segmentation cavity; w.y., white yolk (see Fig. 3). 



During this time a narrow space appears between the center of the disk and 

 the underlying yolk. This space is the beginning of the blastoccel or seg- 

 mentation cavity. Its roof is composed of the smaller central cells; its floor 

 is the yolk, and around its margin it is walled in by the larger partially seg- 

 mented cells and yolk (Fig. 39). If the living blastoderm is observed from 

 above the area over the blastoccel appears clear and is called the area pellu- 

 cida; the area peripheral to the blastoccel appears opaque and is known as 

 the area opaca. 



^ / It is not difficult at this time to make a comparison between the develop- 

 ing hen's egg and the eggs of the frog and Amphioxus. Obviously the stage 

 described above corresponds to the blastula. The small cells forming the 

 roof of the blastoccel are homologous with the micromeres in the frog's 

 blastula; the yolk mass, which in the bird's egg remains wholly unsegmented 

 except around the margin of the blastoderm, is comparable with the macro- 

 meres; the partially segmented cells of the margin of the disk probably corre- 

 sponds to the transition zone which was designated as the germ ring. In 



