THE ORIGIN AND DEVELOPMENT OF THE LYMPHATIC 



SYSTEM. 1 



By FLORENCE R. SABIN. 



(From the Anatomical Laboratory of the Johns Hopkins University, 



Baltimore.) 



I. INTRODUCTION. 

 1. Morphology of the Vascular System, the Axgioblast. 



Until the past few years our knowledge of the morphology of the 

 lymphatic system was in a very unsatisfactory state. Our con- 

 ceptions of such main questions as the origin of the first lymphatics, 

 the time of their appearance, the relations of the lymph hearts 

 and sacs of the amphibia to the lymph glands and duets of higher 

 forms, the relation of the lymphatics to serous cavities and to the 

 various forms of tissue spaces, and the development of the lym- 

 phatic vessels within an organ have heen so vaguely understood 

 that the opinions of no two investigators approached agreement. 



A clear conception of any system must be based on an under- 

 standing of its fundamental morphology, as, for example, the 

 morphology of the arterial system was placed upon a satisfactory 

 basis by The discovery and comparison of the aortic arches in dif- 

 ferent animals. An example more closely in touch with our knowl- 

 edge of the lymphatic system is the study of the fundamental 

 morphology of the blood vascular system. Our knowledge of this 

 is in the process of being built up. but a foundation has been laid 

 for the clear understanding of the vascular system in a series of 

 discoveries. The first and most fundamental of these is that the 

 blood vessels arise as blood islands in the extraembryonal mem- 

 branes. The earlier embryologists on the other hand believed that 

 the first vessels were spaces without walls, which the pressure of 

 the circulating blood hollowed out. The best description of the 



1 Aided by a grant from the Baltimore Association for the Promotion 

 of University Education of Women and by the Carnegie Institution of 

 Washington. 



