287 



cells compose the central part of the balancer. A large blood capillary 

 appears twice in the section. (The circulating blood is visible in the 

 suckers at an early stage just as clearly as in the external gills.) 

 Fig. 5 shows a transverse section of a balancer of a 7,8 mm embryo, 

 about the same stage as Fig. 4. The basement membrane (b.m) lies 

 closely against the bases of the epithelial cells. It is a collagenous 



ec 



Fig. 6. Transverse section through a 9 mm embryo, passing through the anterior part 

 of the base of the right balancer, b. m basement membrane, b. p part of basement 

 membrane extending down into mesenchyme, c cartilage, ec ectoderm. ^505. 



noes 



structure as shown by staining 

 with Mallory's aniline blue, 

 Van Gibson's stain, and Wei- 

 gert's elastic tissue stain. 

 Fig. 6 shows a transverse 



Fig. 7. Transverse section of the 

 right balancer of a 9 mm embryo 

 (same embryo as shown in Fig. 6). 

 b. m basement membrane, ec ecto- 

 derm, vies mesoderm. X ^^0. 



section through the anterior part of the base of the balancer of an 

 older embryo. The basement membrane {h.m) extends deep down 

 into the mesenchyme. This fact together with its position in the 

 earlier stages (Fig. 3) and its collagenous composition indicates that 

 it is of mesenchymal origin, though it is probable that the ectoderm has 

 something to do with its formation. Fig. 7 is a transverse section 

 through the central part of the balancer shown in Fig. 6. The mes- 



