ORIGIN OF VASCULAR TISSUES 65 



Sections of such fragments displayed surprisingly normal con- 

 ditions. The neural grooves would often close normally. If the 

 incision E-F (fig. 1) were made posterior to the anterior intesti- 

 nal portal, the flat blastoderm would often concresce to form a 

 tube. The sections often contained coelomic cavities appar- 

 ently farther anteriorly than one should expect to find in the 

 normal embryo. Above all, such fragments generally contained 

 blood-vessels in various stages of development. 



A very typical picture, frequently obtained in head-meroplasts 

 severed at the anterior intestinal portal is that seen in figure 



11. Here in the fore-brain region the neural folds have failed 

 to meet each other. A head-coelom has developed. On the 

 right side of the figure there is a rather large endothelial cavity 

 with a smaller one below it. Such structures are absent on the 

 opposite side in this particular plane of section. 



Since some of the figures of this paper have been previously 

 described (52) they need not be dwelt on at great length. With- 

 out giving a great deal of evidence concerning the origin of the 

 earliest prevascular cells, I stated (52, p. 334) that ''between 

 the base of the coelomic 'pouch' and the pharyngeal entoderm, 

 rounded or cuboidal cells become proliferated. Their point of 

 origin is in most cases between the base of the coelomic 'pouch' 

 and the pharyngeal entoderm where it is difficult to determine 

 which of these two epithelia is of primary importance in such 

 cell-proliferation." The cells in question are shown in figure 



12. In figure 13, it will be seen that ventral and lateral to the 

 pharynx such cells have arranged themselves into a parenchy- 

 matous complex which merges dorsally into the indifferent 

 mesenchyme. At the time when this figure was first published, 

 I was inclined to believe that such a condition represented an 

 essential stage in the formation of endothelium in this region, 

 and hinted that the resolution of such a complex into endothelial 

 tubes might be closely related to the accumulation of plasma in 

 its meshes. My more recent experiments lead me to believe 

 that such a condition may not necessarily be passed through by 

 the tissue in this region; that the tubes may be formed by a 

 more direct and active manner, in which isolated vesicles are 



THE AMERICAN JOUUMAL OF ANATOMY, VOL. 21, NO. 1 



