248 MARY BLOUNT. 



compact mass with the germ wall, which, like a thickened border, 

 rests upon the yolk. This thickened border also receives the 

 name Randwulst and bourrelet blastodermiqae. 



I would describe the margin of the blastoderm (Fig. 16) not 

 as a region where the upper and under germ layers go over into 

 each other, but as a syncytial region out of which two layers of cells 

 differentiate centrally. 



Through each period of development leading up to this stage 

 (as is shown by the series of figures in this paper), the periblast 

 nuclei keep ahead of the advancing margin of the blastodisc. 

 They multiply and a part of them are used up in the cells that 

 are continually being added to the margin. Such a nucleus in 

 advance of the margin of the blastodisc is shown at the left of 



p.n. p.n. 



Fig. 16. The germ wall in transverse section through the center of the blastoderm 

 of a pigeon's egg, 8:10 A. M., 36 hours after fertilization and 6 hours before laying. 

 p.n, periblast nuclei. They were not all found in this section, but were recon- 

 structed from five successive sections. There are six other periblast nuclei in this 

 half of the section, but in positions central to the limits of the figure. The right 

 hand side of the figure is toward the center of the blastoderm. 



Fig. 1 6. It is in the periblast. It is not enclosed in a cell — i. e., 

 it is not separated by a cleavage plane from the periblast which 

 extends further peripherally — other periblast nuclei are below 

 the margin. The thickened-up character of the margin is due to 

 the upward differentiation of cells from the periblast. It is not a 

 region where the blastodisc is deepened by the opening of the 

 lower layer of cells to send protoplasmic processes into the white 

 yolk. The cells of this region, which are open below, are so 

 because they have not yet, in the process of their differentiation out 

 of the periblast, become closed. Large nucleated masses differen- 

 tiate upward from the periblast. These masses become multi- 

 nucleate, and finally divide up into several cells. As this region 

 becomes older, that is, as it is left behind while the margin of the 



