Hemopoiesis in the Mongoose Embryo. 309 



completely filled with hemoblast-like cells similar to those of the aortic 

 clusters, many of which show degeneration stages, principally a 

 karyorrhexis. But the presence of these cells need not be interpreted 

 as the result of the liberation of a toxin by the atroph^'ing ramus. It 

 seems more reasonable to suppose that the ramus contained a cluster 

 of normal hemoblasts as the result of a normal hemogenic capacity 

 on the part of the endothelium of the vessel, which cluster, in conse- 

 quence of the atrophy and coincident constriction of the vessel, came 

 to occlude the lumen and ultimately to suffer a resultant degeneration. 

 In other words, the occlusion of the lumen of the vessel and the kary- 

 orrhexis of the included hemoblasts are more probably secondary effects 

 of the atrophy of the ramus than that the presence of the cells in the 

 lumen is the result of a toxin formed by the degeneration of the vessels 

 and the included cells and operating as a desquamating stimulant upon 

 the endothelial cells of the vessel. 



The most damaging countervailing evidence, however, to the inter- 

 pretation that the endothelial desquamation products are the result 

 of the action of toxins produced by degenerating blood-vessels and 

 blood-cells, consists in the presence (in the 10-mm. pig embryo) of 

 endothelial cell-clusters of hemoblasts, deep within the superior 

 mesenteric artery (that is, in the middle third of its extent), a level 

 which shows no other signs of atrophy or degeneration and which 

 suffers no subsequent change in a possible farther caudal progression 

 of the vessel to its definitive point of attachment to the aorta. Also in 

 the mongoose embryos and in turtle embryos, the endothelium of the 

 superior mesenteric artery is especially active in liberating intravascular 

 cellular elements. 



In a study of the aortic cell-clusters in loggerhead-turtle embryos 

 my attention was arrested by peculiar formations in the inferior vena 

 cava at the level of fusion of the subcardinal veins of a specimen of 

 the twelfth day of incubation. These formations consisted of encap- 

 sulated spheroidal masses of hemoblast-like cells, and long strings of 

 hemoblasts attached to the endothelium and extending free into the 

 lumen of the vessel. In the case of the encapsulated groups the 

 capsule consists of an endothelioid membrane of greatly flattened cells. 

 The inclosed cells are very similar to those of the naked cell-clusters of 

 the aorta and the superior mesenteric artery. The structure is com- 

 parable to Emmel's figure 5, which represents a similar formation in 

 the aorta of a 12 mm. pig embryo. In one instance the mass was con- 

 tinuous with an underlying loose mesenchyma which appeared to be 

 differentiating into hemoblasts. I incline to interpret this structure 

 in terms of my figure 5. If the invaginated area of endothelium had 

 included a considerable portion of the subjacent vascularizing mesen- 

 chyma, then the mesenchyma might have outstripped the endothelium 

 in the process of differentiating into hemoblasts, and so forced, through 



