DEPARTMENT OF EMBRYOLOGY. 75 



Doan, working together, have found, however, that the marrow of rabbits can 

 be depleted of its myelocytes by the intravenous administration of large doses 

 of dead typhoid bacilli. In this way they have been able to demonstrate the 

 intersinusoidal capillary plexus for mammals and have shown the probabihty 

 that in the rabbit, as in the pigeon, red cells arise within the capillaries by 

 proliferation of the endothelium, and that they enter the circulation through 

 the capillaries into the sinusoids of the marrow. 



Entrance of Small Lymphocytes into the Blood-Stream. 

 In my last report mention was made of the experiments of Dr. F. C. Lee, 

 in which he ligated the thoracic duct by an intra-thoracic method, and of the 

 lymphatico-venous anastomoses which he found to occur between the thoracic 

 duct and the azygos vein. Continuing this work, he has studied the changes 

 in the number of small lymphocytes in the blood-stream following such ligation 

 in the cat, it being fairly well established that the thoracic duct is an im- 

 portant avenue for the entrance of small lymphocytes into the blood. Dr. 

 Lee finds that ligation of this duct produces an immediate decrease, to the 

 extent of 56 per cent, in the number of small lymphocytes, but that the pre- 

 operative level is again reached at about the end of three weeks. Having 

 demonstrated the collateral circulation that occurs following ligation, it 

 seems probable that we have here the explanation of the means by which the 

 gradual return of the number of small lymphocytes to the preligation level 

 takes place. 



Development op the Aortic-Arch System. 



As a result of the studies of Dr. E. D. Congdon and Dr. C. H. Heuser, we 

 are now finally in a position to discard the Rathke diagram, which for over 70 

 years has dominated descriptions of the development of the aortic arches. 

 Owing to the abundance of their material and the improvements in their 

 technical methods, Dr. Congdon, working with human embryos, and Dr. 

 Heuser with pig embryos, have been able to demonstrate every stage in the 

 transformation of the aortic-arch system. It becomes clear from their obser- 

 vations that these vessels are not so much concerning themselves with carry- 

 ing out a program of recapitulation as with following out a procedure in angio- 

 genesis that is exactly like that prevailing in all other parts of the body. 



There is first the differentiation of a given amount of endothelium and 

 plasma. The endothehum is for the most part plexiform, but may exist as 

 simple channels conforming to the available space and the character of 

 adjacent structures. In the beginning its pattern bears no relation to any 

 hydrostatic circulatory requirements, and this phase in its development can 

 only be regarded as the laying down of an endothelial bed. As circulation 

 begins, the endothelial bed is gradually modified into a system of suitable 

 channels of supply and drainage for the surrounding structures, and one can 

 then begin to speak of arteries, veins, and capillaries. These are simple 

 while the structures are simple, and become more and more complicated as 

 the latter become more elaborate. The pattern of the early blood-vessels 

 shows every evidence of being almost entirely determined by the environment. 

 Dr. Congdon and Dr. Heuser, by a remarkable series of adaptations, have 

 given us a better description than we have thus far had of how the primitive 

 pattern is modified into the final arteries and veins which are met in the adult, 

 and they have also given a basis for a better understanding of the various 



