262 CHARLES R. STOCKARD 



The later development of the intermediate cell mass is found 

 to proceed in almost exactly the manner described by Swaen 

 and Brachet ('01, '04). This mesenchymal mass of cells is at 

 first of an indefinite type lying between the notochord above 

 and the intestine below and being flanked on either side by the 

 primary nephric ducts. The first notable differentiation of the 

 intermediate mass in the normal embrj'o begins shortly previous 

 to the establishment of a heart beat. In an experimental em- 

 bryo of seventy-two hours old, that is one in which the heart 

 was just about ready to begin beating, figures 31 and 32 show the 

 condition of the intermediate cell mass in cross section. 



In figure 31, which is the extreme anterior end of the mass and, 

 therefore, less well differentiated than the more posterior regions, 

 the cells are seen to possess large round nuclei differing but 

 slightly from the nuclei of the surrounding cells and those of 

 the epithelium of the Wolffin ducts. The mass of cells is com- 

 pletely unsurrounded by endothelium, and I agree entirely with 

 Swaen and Brachet that the central cells go to form the red blood 

 corpuscles while the cells about the periphery of this mass form 

 the vascular endothelium. 



Figure 32 illustrates a section through a more posterior region 

 of the same embryo, the intermediate mass is seen to be much 

 further differentiated. The cells are here typical early erythro- 

 blasts and many are observed to be in active mitosis. The cells 

 in the mass are becoming dissociated so that they are no longer so 

 densely packed as in the section through the anterior region. 

 This section is posterior to the ends of the Wolffian ducts, as 

 well as the closed intestine and beneath the cell mass is shown 

 the periblast over the yolk. 



On tracing the series still further caudad, we find the indefinite 



VPy^ cell mass end Knospe described by Marcus ('05), figure 33. 



This is a ventral cellular mass into which leads the notochord, 

 intermediate cell mass and end of the endoderm. In other 

 words, this mass would seem to represent the end bud at the dor- 

 sal blastopore lip, as if it were the point from which differ- 

 entiation had taken place or from which the layers had grown 

 forward. 



