THE DEVELOPMENT OF THE SUBCUTANEOUS VASCULAR PLEXUS 

 IN THE HEAD OF THE HUMAN EMBRYO. 



When the work of the last ten to fifteen years is analyzed, it becomes clear 

 that the fundamental problem of the vascular system is concerned with the origin 

 and growth of endothelium, since the entire vascular system begins from specific 

 cells which later develop into vessels. These essential cells of the vascular system 

 are the angioblasts of His or the vasoformative cells of Ranvier. With the term 

 "angioblast" was early associated the idea of His that complete differentiation of 

 vasoformative cells takes place in the yolk-sac, these cells later invading the embryo 

 itself. This idea had to be abandoned when it was proved that angioblasts dif- 

 ferentiate within the body of the embryo. In a living preparation of a chick embryo 

 the aorta has been observed by Dr. Sabin to differentiate in situ, and it now seems 

 probable that many of the primary veins differentiate in the same way. 



In a consideration of the origin and growth of endothelium, one of the most 

 important points to be determined is the length of the period during which angio- 

 blasts continue to differentiate from undifferentiated mesenchyme. With such a 

 consideration in mind, the study of the development of the vascular system of the 

 head in the human embryo becomes significant. There are two main vascular 

 plexuses to be observed in the head: (1) the meningeal, which first appears in 

 embryos of about 4 mm., and (2) the subcutaneous plexus, which appears at about 

 the 20-mm. stage. There is thus a marked difference in the extent of differentiation 

 of the embryo itself. From the meningeal plexus develop the vessels of the central 

 nervous system, the dorsal sinuses, and the vessels of the skull, while from the 

 subcutaneous plexus develop the vessels of the skin and of the head-musculature. 

 The two unite in common with the vessels of the neck, but on the sides and vault of 

 the head the two systems are completely separated by the developing membranous 

 skull. The subcutaneous plexus, being thus isolated and spread out as a thin sheet 

 that can be examined in a total mount, constitutes a particularly valuable field for 

 study of angioblastic differentiation in this relatively late period of embryonic life. 



The material used for the study of the subcutaneous plexus of the head con- 

 sisted of serial sections of human embryos in the Carnegie Collection and of total 

 mounts of skin flaps from the area of the head plexus. Before making the dissection, 

 the fixed embryo was studied in the gross, whenever possible, and the general plan 

 of the plexus was determined. Several small skin flaps were then carefully dis- 

 sected from the sides of the head and examined, first unstained and later after 

 staining. In several instances tangential serial sections were cut. The principal 

 stains used for the total mounts were alum cochineal, and hematoxylin, either in 

 combination with eosin, aurantia, and orange G or with eosin alone. Wright's 

 blood-stain was used for some of the tangential sections. Total mounts were 

 found to have distinct advantages, since they afforded an opportunity to study a 



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