ORIGIN OF VASCULAR TISSUES 67 



A condition similar to those described in embryo Type I, 

 No. 31 (figs. 14 cand 21) is seen in Type I, No. 122 (fig. 22). In 

 this embryo the endothelial tubes have a median connection, 

 but in this section their cavities are distinct. Posteriorly the 

 cavities themselves communicate. Despite the great differences 

 in the development of the blastoderms at the time of operation, 

 Type I, No. 31 and Type I, No. 122 show very great similarity 

 of structure; the conditions in the former are shown in figure 19. 

 This figure is a photograph of a cross-section of the blastoderm 

 (exclusive of the meroplast) which was removed at the time of 

 operation. In this operation the incisions A-B and C-D extended 

 somewhat posterior to the line E-F. The plane of section lies 

 between the line E-F and an imaginary line A-C (fig. 1). Thus 

 we obtain a section showing the body axis, discontinuities corre- 

 sponding to the longitudinal incisions A-B and C-D, and lateral 

 to these, the outlying blastoderm. Such a section records both 

 the position of the longitudinal incisions and the conditions of 

 vascular development in the body axis as well as the extra- 

 embryonic area. This section shows the greatest mesial extent 

 of the yolk-sac vascular tissues which are in continuity (fig. 19). 

 Lying here and there, mesial to these vessels, are occasional 

 cells or groups of cells, apparently in process of separating from 

 the splanchnopleure. These may well be regarded as fore- 

 runners of vascular tissue. No vessel fulfilling the requirements 

 of a sprout of 'angioblast' had yet approached the median axis. 

 The tissue between the two incisions (fig. 19) represents the 

 middle or anterior part of the hind-brain region. It would 

 perhaps have been the first part of the body to have been 

 'invaded' by vitelhne capillaries, if invasion had been the true 

 process of vascularization. The longitudinal incision repre- 

 sented on the right side of figure 20 was not parallel to the 

 median axis, but approached it anteriorly. 



Figure 20 is a photograph of a transverse section of the blasto- 

 derm belonging to the meroplast of which figure 22 is a section. 

 The plane of figure 20 is only a short distance behind the line 

 E-F of figure 1. The points A and C in this case He in the 

 line E-F; hence the effects of the longitudinal incisions are not 



