88 J. THOS. PATTERSON. 



is well laid down, and even the large vessels are seen to pass 

 inwards to the center of the pellucid area. Attempts to develop 

 such blastoderms to the seventy-second hour have failed, from 

 which fact it would seem that after about the forty-eighth hour 

 of incubation the expansion of the vascular area over the yolk 

 is dependent on the propulsion of the newly formed blood-cells 

 through the vessels, that is, on the development of the embryo 

 with its heart. 



Discussion. The results obtained in experiments I. and II. 

 make it practically certain that in the absence of the primitive 

 streak mesoblast the germ-wall does not give rise to vascular 

 mesoblast. Hence, the view that the mesoblast of the area opaca 

 is split off from the germ-wall is doubtless incorrect. There is 

 good evidence, of course, to show that the mesoblast in this area 

 must receive material from the germ-wall. Lillie has pointed 

 out that the mesoderm cells of the area pellucida are void 

 of yolk-granules, while those of the blood-islands " contain yolk- 

 granules of precisely the same character as those of the germ- 

 wall." He states, " therefore, either the blood-islands are derived 

 from the cells of the germ-wall, or cells of the mesoderm growing 

 out over the germ-wall burrow into the latter, engulf yolk- 

 spheres, and reappear in masses as blood-islands.'' 1 The former 

 of these alternatives is disproved by the result obtained in experi- 

 ment I. ; the latter, therefore, must be regarded as the correct 

 view. In this " feeding " of the mesoderm cells on the germ-wall 

 material during the formation of blood-islands, we doubtless have 

 an explanation for Riickert's 2 contention, viz., that in blood-island 

 formation the primitive streak mesoderm receives elements from 

 the germ-wall ; for it is difficult to imagine how it would be 

 possible for the mesoderm cells to take up germ-wall material 

 en masse without also including many elements (cells) therein 

 contained. However, the participation of the germ-wall with 

 the primitive streak mesoderm in the development of vascular 

 tissue, must be regarded as secondary (perhaps merely furnishing 

 nutriment), and the power to differentiate blood-islands, there- 

 Millie, F. R., "The Development of the Chick," New York, 1908, p. 89. 

 2 Riickert, J., " Entwickelung der extra-embryonalen Gefasse der Vogel," 

 " Handbuch der vergl. \v. exp. Entw. lehre der Wirbelthiere," Bd. I., T. i, 

 1906. 



