ORIGIN OF BLOOD AND ENDOTHELIUM ' 49 



is completely disintegrated and has disappeared, and the very 

 degenerate small erythrocytes are now intermixed with mesen- 

 chymal cells. In a shghtly older embryo, all of these blood 

 cells have disappeared within the tissue as if the invading mesen- 

 chymal cells had really assimilated or destroyed the old blood 

 cells. 



It is thus seen that in these non-circulating individuals, al- 

 though red blood cells arise in a perfectly normal fashion and 

 differentiate as completely as in the control embryos, yet they 

 are not capable of maintaining their fully developed condition. 

 Sooner or later they undergo degeneration and finally are com- 

 pletely absent from the body of the embryo. 



It is noticed in all cases that very soon after the erythroblasts 

 become completely surrounded by endothelium, they gradually 

 lose their power of multiplication and then differentiate into 

 typical erythrocytes. Before the vascular wall has completely 

 enclosed the erythroblasts, all groups often show many cells 

 in active mitosis, and as I shall bring out below those spaces in 

 which blood cells multiply both in the embryo and in the adult 

 are spaces not completely surrounded by vascular endothelium. 



In examining figures 40 and 42, it may be of interest to note 

 that the erythrocytes in figure 40 are the typical ichthyoid 

 type of Minot ('11), while those in figure 42 are what Minot 

 would describe or term, the sauroid type; that is, erythrocytes 

 in which the nucleus has become shghtly more degenerate or 

 more densely staining than in the ichthyoid type and in which 

 the cell body is smaller. This sauroid type of corpuscles Minot 

 has designated as being characteristic of reptiles and amphib- 

 ians, and the condition in these embryos without a circulation 

 indicates the very artificial nature of the proposed classification 

 of Minot. The cells are, of course, ichthyoid but are degenerate 

 and, therefore, assume the 'sauroid type.' 



It is difficult for one to believe that all of the functioning 

 erythrocytes in the amphibians and reptiles really have a degen- 

 erate nucleus for the simple fact that mammals have blood 

 corpuscles which have completely lost or discarded their nuclei. 

 It must here be remembered that birds are as truly derived from 



