6 WILLIAM A. HILTON. 



yolk in all direction- from a i;reat mass of ce!l> surrounding the 

 bla-topore. extending forward and to a slight extent back of it. 

 Thi- ma-> of cell-, which is continuous cephalad with the nervous 

 system and the- mesoblast, is in this more caudal region an un- 

 ditf'ereniiated -roii]). It is from this that the mesoblast grows 

 out io meet that which has been formed on each side of the 

 iiotorhord (Fig. 8). The place where the mesoblast from the 

 cephalic region and that from this caudal mass meet is ofu-n 

 distinguishable because of a difference in the staining character 

 of tin 1 cells. In this late stage when the body of the embryo is 



FIG. 8. Cross section through the caudal end of a late medullary plate stage, 

 showing the undifferentiated cell mass, and the small size of the archenteron. 



outlined, the cavity of the archenteron is much reduced in size 

 (Fig. 8). This caudal mass is made up from cells on all sides 

 of the blastopore, but especially cephalad of it and evidently 

 represents in large part, the small cells or their progeny which 

 with the narrowing of the blastopore have been brought to the 

 dorsal side of the yolk mass. 



In the study of these stages in Desmognathus, one is impressed 

 with the fact that fluids within the egg between the blastomeres 

 have more significance than has been generally recognized and it 

 seems possible that this fluid varying with the condition of 

 development, may have considerable importance in the change 

 of form and po-hion of the cells. At times this liquid seems to 

 exert no active influence on the size or condition of the cavitie- 

 or -hapes of the cells, and at others it seems to definitely force 

 the cells into an even layer. For instance in early segmentation 

 Stages, those from the yolk mass and other surrounding cells 

 round oil to\\.ird the bl.i-tonele and tend to fill it in, while at 

 other time-, as in late -a-trula Stages, the edij.es of the cell- 



