140 ADAM M. MILLER 



resemble the latter in every respect except size. These are un- 

 doubtedly microcytes, the earlier stages being microblasts. 



After the middle of the fifth day the masses of developing 

 blood cells increase rapidly in size by proliferation of the com- 

 ponent cells and continued differentiation of the branching ele- 

 ments of the mesenchymal syncytium. They increase in number 

 also by the same processes of differentiation in other localities. 

 Up to about the beginning of the sixth day all the developing 

 blood cells in question remain extravascular, that is, free in the 

 intercellular mesenchymal spaces which are not lined by endo- 

 thelium. 



By the end of the seventh day the aggregations of cells reach 

 the height of their development. At this time they extend in 

 two main lines from the level of the aortic arches along the dorsal 

 aortic roots to the confluence of the latter to form the aorta (figs. 

 6, 20 and 22, 16). At this level, or a little further caudal, the two 

 main lines unite to form a single line which extends about to the 

 level of the superior mesenteric artery (figs. 7, 8, 10, 20 and 22, 16). 

 At their greatest development the larger groups together form an 

 almost continuous mass of the cells which in places is greater in 

 diameter than the aorta. Usually there are also numerous smaller 

 outlying groups which belong to the same general line (figs. 7 

 and 8). 



While the main lines above described are established in all 

 the embryos examined, yet there is a wide range of variability 

 in the form of the groups and their arrangement in the lines. 

 In most of the embryos examined a large mass of cells or a collec- 

 tion of smaller groups had developed in the region dorsal to the 

 aortic roots and esophagus (figs. 6 and 22, 16a). One of the most 

 interesting and one of the most important features of the masses 

 of developing blood cells is the fact that the main lines in their 

 general arrangement correspond with the lines of the thoracic 

 duct (figs. 20, 22 and 24, 16, 17 ,17a). 



To anticipate, it may be stated here that as the multiple anlagen 

 of the thoracic duct develop the masses of developing blood cells 

 are in a large part included within them, and thus become strictly 

 intravascular (see fig. 10, 16, 17). The details of this process will 



