2 Florence B. Sabin. 



origin of blood islands and the best analysis of the meaning of this 

 discovery is to be found in two works of His, " Untersuchungen 

 ueber die erste Anlage des Wirbeltierleibes " in 1868 (pp. 95-103) 

 and in " Lecithoblast und Angioblast" in 1900 (pp. 268-295). The 

 discovery of blood islands, however, dates back to the work of 

 Wolff (154) and Pander (101), who introduced the name, and 

 perhaps no one subject in embryology has a more extensive litera- 

 ture. Certainly a most interesting account of the development of 

 the vascular problem can be followed through the pages of von 

 Baer (7), Prevost and Lebert (108), Kemak (124-125), Eeichert 

 (121-123), Koelliker (67-71) and Thoma (148). The second 

 great advance was the discovery that blood vessels throughout are 

 lined by endothelium (Hoyer 48), which followed soon after the 

 corresponding discovery in lymphatics by von Eecklinghausen. The 

 third discovery involves the proof that blood vessels grow by the 

 sprouting of their endothelium, Prevost and Lebert (108), His 

 (47), Eouget (126) and Arnold (5) ; the fourth that the main 

 vessels of the body wall, including the posterior part of the aorta 

 (Evans 33-35), even the anterior part of the aorta and lateral heart 

 anlagen, arise as a capillary plexus or as solid angioblast cords 

 (Bremer 15), which invade the body from the extraembryonal mem- 

 branes. Finally a complete conception of the development of the 

 vascular system is based on the theory that the blood vessels of the 

 extraembryonal membranes invade the body wall (His 47), and 

 that within the body wall these capillaries of endothelium gradu- 

 ally invade or spread over the body so that there are definite vascu- 

 lar and non-vascular layers and zones. His (47) discovered the fact 

 that blood vessels grow into the central nervous system ; but the theory 

 of vascular and non- vascular zones which is essential to an understand- 

 ing of the development of the vascular system we owe to Mall (80-87). 

 It has been worked out by his pupils, notably Evans (33-35). Its 

 meaning can be grasped by studying figs. 4, 5 and 6 in connection 

 with fig. 437 (Evans 35), which all show that the skin dorsal to the 

 central nervous system is a non-vascular zone, which for a long 

 time is not reached by the blood vessels in the centrifugal growth. 

 This series of discoveries, notwithstanding the gaps and uncer- 

 tainties in our knowledge of the early stages which were well 

 brought out by Minot (98, pp. 483-485) and by Evans (35, pp. 551- 

 567) in 1911, uncertainties in part since removed by the work of 



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