120 RITTER 



notochordal pockets are pierced by the most anterior branchial 

 orifices. But of course this is a region of transition. The 

 skeletal crura gradually shade off into strands and thin laminae 

 of skeletal substance ; the notochordal pockets are gradually 

 lost, and the vacuolated notochordal tissue gradually disappears. 

 Attention should be directed to the fact that the tissue of the 

 median ridge of the notochord (w. r.) has more the character, 

 histologically, of the general esophageal wall than have the 

 pockets, and that this comes to constitute a larger and larger 

 proportion of the entire esophageal notochord as we pass pos- 

 teriorly. It is nevertheless true that the outermost layer of the 

 ridge is to the very last devoid of nuclei, and is more highly 

 vacuolated than is the epithelium in other parts of the esopha- 

 gus. The middle portion of this ridge continues on through- 

 out the pharynx as the c^ibranchial hand. This latter organ is 

 unusually prominent in the present species, and its histological 

 structure resembles closely the notochordal tissue where this 

 reaches its best expression. Its cells are large and vacuolar, 

 and its nuclei are few (fig. 8). Its most characteristic develop- 

 ment is reached at some little distance behind the beginning of 

 the pharynx, but I am inclined to believe that in this species a 

 mid-dorsal hand of enteric zvall differentiated into chordoid tissue 

 extends without interruption throughout the collar and pharyn- 

 geal regions. It is impossible to discuss in the present paper 

 the theoretical bearings of this proposition. Indeed, it may be 

 as well that it is impossible. However, a somewhat fuller state- 

 ment of the facts on which it rests is undoubtedly due. Spengel 

 has pointed out^ that in the genus Balanoglossus the epibranchial 

 band is a thickened strip of the mid-dorsal wall of the phar3-nx 

 that is not encroached upon by the series of branchia?. It 

 is, then, interposed between the dorsal portions of the two 

 branchial series. In the other genera, notably Plychordera, on 

 the other hand, the branchial series of the opposite sides come 

 into actual contact dorsally, so that none of the enteric wall 

 is retained between them. The former condition he regards 



' See, for example, his description and figures of the structure in B. kupjferi 

 and B. canadensis, and his diagrammatic text figures on page 545 of his mono- 

 graph. 



