86 



J. THOS. PATTERSON. 



I, B}. It gives the same general result, in that the lateral ex- 

 pansion of the vascular area has been retarded (Fig. 5). Some- 

 times, however, as in the case of the right side of this embryo, 

 the lateral margins of the vena terminalis within the bay grow 

 towards each other and finally fuse, thus leaving the scar of the 

 injury within the margin of the vascular area. In the majority 

 of cases, however, this does not occur. 



Experiment III. Another experimental method of demon- 

 strating that the primitive streak mesoblast is the source of the 

 vascular mesoblast of the area opaca, consists in destroying a 

 given portion of the early primitive streak and then noting the 

 effect on the development of the vascular area. A few of these 

 experiments may be briefly mentioned. If the posterior part 



ec 



urn yse 



FIG. 4. This is the central part of the section taken in the plane A-B, 

 Fig. 3. vt, vena terminalis : ec, ectoderm ; yse, yolk-sac entoderm ; pm, parietal 

 layer of mesoderm ; Tin. vascular layer of mesoderm ; gis. 1 , portion of the germ- 

 wall lying between the two sections of the vena terminalis. Note that- it is 

 not differentiated into vascular mesoblast. 



of the early primitive streak, together with a small region of the 

 adjacent area opaca, 1 be destroyed, the posterior end of the 

 vascular area is wanting in the later embryo; but the anterior 

 parts of the area are entirely normal. Again, if the anterior tip 

 of the early primitive streak be destroyed, the later embryo 

 shows a deficiency in the anterior part of the vascular area. 

 Finally, if the posterior part only of the primitive streak be 

 destroyed (Fig. I, C), the later embryo lacks the lateral parts 

 of the vascular area which lie opposite the injuries (Fig. 6). In 

 other words, the destruction of any portion of the early primitive 

 streak results in producing a deficiency in the part of the vascular 

 area lying at the level of the injury. 



1 This is done in order to destroy that part of the primitive streak material 

 which overlaps the area opaca at the posterior border of the area pellucida. 



