ORIGIN OF VASCULAR TISSUES 113 



can orient himself with reference to figure 87, which is the same 

 section of the meroplast somewhat enlarged. The orientation 

 of figure 88 is similar to that of this figure. Examination of 

 figure 87 shows that the plane of section is through an otocyst, 

 probably the left. There is a very thin nervous epithelium partly 

 surrounded by a cartilaginous otic capsule. Laterally located is 

 evidently the anlage of a semi-circular canal. Strangely enough 

 the ganglionic mass accompanying this otocyst lies lateral to it. 

 The tissue on the right side of the otocyst is a rather dense 

 mesenchyme, some of the cells of which are spindle-shaped and 

 massed into rather fibrous complexes. This region to the right 

 of the otocyst represents the median axis of the transplanted 

 tissue. This section contains neither endothelial tissue nor 

 corpuscles. This is true of the remaining posterior portion of 

 the meroplast. The other figures shown are from planes ante- 

 rior to this. 



It will be noted that the yolk-mesoderm Ventral' to this mero- 

 plast has become fibrous. Higher magnification of this tissue 

 (fig. 87) reveals the fact that there are numerous muscle fibers. 

 These seem entirely comparable to those always observed ventral 

 to the anterior portion of the normal embryonic body. It seems 

 that the presence of this anterior axial tissue brings about the 

 formation of muscle fibers. These are, in all probability, in situ 

 differentiations of the yolk-sac mesoderm and not migrations 

 from the mesoderm of the transplanted tissue. This point is dif- 

 ficult to prove. 



The most anterior section figured may now be considered. 

 The plane of section (fig. 88) is near the anterior extremity of 

 the meroplast. The axial portion is solid cartilage. To the 

 left of this the tissue contains some ganglionic material and 

 mesenchjane, which lies anterior to the otocyst region of figure 

 87. Within the next few sections anterior, this region on the 

 left entirely disappears, leaving the cartilaginous axis alone in 

 section. The region between the solid cartilage of figure 87 and 

 the axial portion of figure 88 contains mesenchyme cells differen- 

 tiating into erythrocytes. The cells are rather scattered and 

 none of their groups is large. Figure 89 shows a section taken 



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



