98 CARNEGIE INSTITUTION OF WASHINGTON. 



are going on in the area pellucida they can be watched with much 

 clearness, because here the cells are so thin that they can be focused 

 through with ease. 



In studying these early living blastoderms, Dr. Sabin has made the 

 important observation that there are definite cycles of cell division in 

 the vascular system, as well as in the nervous system and in the 

 endoderm. Recognition of these cycles depends on changes in the cyto- 

 plasm, rendering it more refractive at the time just preceding cell di- 

 vision. The cytoplasm is more basophilic during this phase, but this 

 reaction to dyes in fixed specimens is not so brilliant as the change 

 in refractivity in the living form. It is not certain whether or not the 

 mesoderm undergoes these same cycles of cell division. In the case of 

 the vascular system in the early stages, all of the angioblasts of a given 

 area, or all of the primitive blood cells, can be seen dividing at the same 

 time. As a result, a certain interesting sequence may be made out in 

 following the development of the vascular system in such preparations. 

 Vessels first appear as solid masses at the outer margin of the area 

 opaca. These masses form a plexus which soon becomes a plexus of 

 hollow vessels, and while blood is forming within these marginal vessels 

 new angioblasts differentiate in the inner half of the area opaca. Again, 

 the same process takes place in the inner and outer margins of the area 

 pellucida, making four different vascular zones in the embryonic mem- 

 branes. By the time the chick has 10 somites, vessels have differen- 

 tiated over all the membranes, there is a simple aorta, and the outer 

 part of the system is filling up with blood. At this stage the heart 

 begins to beat, but the beating does not propel the blood until the 

 stage of about 16 somites, when all of the blood in the outer rim of 

 vessels is swept into circulation and a new cycle of young blood islands 

 starts within these same vessels. As a consequence of these cycles, 

 which sweep across the membranes again and again, all of the phe- 

 nomena, the origin of angioblasts, their transformation into vessels, 

 and the formation of red blood cells, can often be seen in a single sec- 

 tion of a chick in the second half of the second day. 



Two investigations have been completed on the lymphatic system; 

 one by Dr. J. R. Cash, deahng with the lymphatic drainage of the 

 stomach, and the other by Dr. F. L. Reichert, dealing with the fate of 

 the primary lymph sacs of the abdomina,l and pelvic regions. Both of 

 these are based on injections of the lymphatic vessels in embryo pigs in 

 various stages of development. 



The work of Dr. Cash was started two years ago and is now published 

 in completed form. He finds that the lymphatics pass to the stomach 

 by way of its related folds of peritoneum rather than by always follow- 

 ing the course of the blood-vessels. At 28 mm. the vessels which sub- 

 sequently form the right gastric trunk have reached the stomach 

 through the gastro-hepatic ligament. It is from this group that most 



