162 DEVELOPMENT OF VASCULAE PLEXUS IN SCALP OF HUMAN EMBRYO. 



DESCRIPTION OF PLATES. 

 Plate 1. 



Fig. 2. Photograph of a human embryo 23 mm. in length (No. 966), showing the vascular plexus in the subcutaneous 

 tissue of the head in its earliest form. It is characterized by two distinct growth centers, the temporo- 

 frontal and the occipital, from which the vessels gradually spread over the apex of the head. A sharplj 

 defined area between the two growth centers constitutes an angle of retarded growth. X4. 



Fig. 3. Photograph of a human embryo 27.5 mm. in length (No. 2561), showing a later stage of the plexus. The 

 angle of retarded growth is not as prominent and the margin of the plexus appears as a narrower and 

 more well-defined line than that in figure 2. X4. 



Fig. 4. Photograph of a human embryo 36 mm. in length (No. 1591), showing a late stage in the closing in of the 

 plexus. X2. 



Fig. 5. Photograph from a total mount of a piece of the scalp from a human embryo 28 mm. in length (No. 1240a). 

 The varied forms of the growing tips are well shown and the transition from the angioblastic net to the 

 capillary net can easily be followed. X80. 



Fig. 6. Photograph from another portion of the same section as above, showing, under higher magnification, the 

 first and second zones. In the center a long tip from the angioblastic plexus is seen to penetrate the 

 avascular zone. This represents the first step in the differentiation of the mesenchyme into angioblastic 

 tissue. X 150. 



Plate 2. 



Fig. 7. Drawing of a growing tip, showing red blood-cells as they first appear, seen at the edge of the head plexus in 



a human embryo 28 mm. in length (No. 1240a, total mount). The club-shaped cellular mass has an 



indefinite connection with the main angioblastic plexus. X930. 

 Fig. 8. Drawing of a growing tip at the edge of the head plexus in a human embryo 23 mm. in length (No. 966). 



Several well-defined endothelial cells can be made out at the edge of the angioblastic strand, and there 



is a fine filamentous strand at the extreme tip, which appears to be an endothelial process. X930. 

 Fig. 9. Drawing of a growing tip at the edge of the head plexus in a human embryo 28 mm. in length (No. 1240a, 



total mount). Two cells with clear, colorless cytoplasm may be observed. X930. 

 Fig. 10. Drawing of a capillary from the third zone of the head plexus in a human embryo 19 mm. in length (No. 431). 



The capillary is seen to be empty save for three nucleated red blood-cells. X930. 

 Fig. 11. Drawing of a typical growing tip at the edge of the head plexus in a human embryo 26.4 mm. in length 



(No. 1008). X930. 

 Fig. 12. Drawing of a strand of early red cells, containing a slight amount of hemoglobin, and having no apparent 



connection with the main angioblastic plexus. Taken from a total mount of the scalp of a human 



embryo 23 mm. in length (No. 1358/, total mount). X930. 

 Fig. 13. Drawing of a chain of early red cells, similar to that seen in figure 12, showing no connections with the main 



angioblastic plexus. Taken from a total mount of the scalp of a human embryo 23 mm. in length 



(No. 1358/, total mount). X930. 



