84 CARNEGIE INSTITUTION OF WASHINGTON. 



is by the amount and arrangement of its granules. This should be of great 

 advantage clinically, where one is studying conditions in which blood is regen- 

 erating. It will be necessary now to work out the exact stages of develop- 

 ment of the vitally stainable granules in the human embryo, and we can then 

 estimate just how primitive are the young cells found in the circulation for 

 each phase of regeneration. 



The fii'st white cell makes its appearance on the thu-d day of incubation; 

 this is the monocyte or transitional cell. It develops within the vessel from 

 the endothelium, acquiring its characteristic structure before becoming 

 detached from the wall of the vessel. At the same time the clasmatocytes 

 can be seen arising from the endothelium on the outside of the vessels, being 

 identical in form with the monocytes. In other words. Dr. Sabin shows that 

 the monocytes of the blood and the clasmatocytes of the connective tissue are 

 derived from the same epithelium and are identical, except that one is intra- 

 vascular and the other extravascular. They are afterward interchangeable, 

 however, for clasmatocytes have been seen to enter the vessel and monocy1;es 

 to pass out. The granulocyte is derived from the mesoderm outside of the 

 vessels and can be first seen on the third day of incubation. At first it 

 resembles a single angioblast, but as soon as division takes place it can be 

 differentiated; two granulocytes separate, whereas two angioblasts remain 

 together. This whole process and the subsequent differentiation and entrance 

 of the granulocyte into the vessels can be watched in the living preparations. 

 Up to the seventh day there is no evidence of the formation of lymphocytes 

 in the yolk-sac membranes. They are found, however, in the circulating 

 blood on the fourth and fifth days. Their origin must be looked for in the 

 body of the embryo. 



In studying the blood-vessels of the bone-marrow, Mr. C. A. Doan has dis- 

 covered that, in addition to the large, thin-walled venous sinusoids that form 

 the principal functioning vascular bed of the marrow, there is an extensive 

 inter-sinusoidal capillary plexus hitherto unsuspected. This plexus is ap- 

 parently collapsed in its normal state, but is capable of distention under the 

 conditions of the injection technique. A preliminary account of Mr. Doan's 

 observations has appeared and the complete report of his study is now in 

 course of publication. 



Lymphatic Vessels. 



Working in a field in which such notable results have been accomplished by 

 Professor Sabin and Dr. Cunninghan, Dr. F. C. Lee has devised a satisfactory 

 intrathoracic method for the ligation of the thoracic duct in the cat. Animals 

 upon which the operation was performed have lived as long as 66 days, 

 apparently unaffected, and it is thus clearly evident that the integrity of the 

 thoracic duct is not essential to the life of this animal. Where the ligation did 

 not include the periaortic plexus, a collateral lymph circulation was established 

 to the right thoracic duct. Where the ligation was complete, the investigator 

 found well-defined lymphatico-venous anastomoses existing between the 

 thoracic duct and the azygos vein. It is clearly evident, therefore, that these 

 connections must be taken into consideration in any future study of fat 

 absorption. In reviewing what is known regarding the embryology of these 

 structures, Dr. Lee does not find an adequate basis for their explanation. 



