386 



Bashford Dean Memorial Volume 



Text-figure 58. 



Section through the tropeic folds (x 25) of a 210 mm. embryo of Chlamydoselachus, showing 



the keel muscle {\.m.). The section was taken about 15 mm. in front of the pelvis. 



Drawn from a specimen collected in Japan by Dr. Bashford Dean, and now in the American Museum. 



under consideration. It is clearly derived as a simple inpocketing of the ventral muscula' 

 ture of the body wall, in the region where the ventral bundles of the two sides of the 

 body meet. Furthermore, it is segmented after the fashion of the metameric muscles of 

 the body wall — a feature that is entirely lacking in Carman's drawings and is not men' 

 tioned in his text. Earlier stages would be required to show continuity of the muscula' 

 ture in this region. 



Evidence regarding the manner of origin of the keel muscle was obtained by Braus 

 (1898, Fig. 2, pi. XIII) in connection with his studies of the innervation. In this case 

 the depth of the "keel" is remarkable. Braus appUes the term rectus to the thin muscle 

 of the body wall in the region of the ventral mid'line — a muscle which is interrupted by 



MohL iM 



U.oU.uit. 



' Pentpn. ■ u 



K.mUrciM. "■ H.OOL.inl. 



Text-figure 59. 

 Diagrams of sections (all inverted) showing the probable manner of origin of the keel muscle: 

 A, absent in Squalus; B, hypothetical intermediate stage; C, as in adult Chlamydoselachus. 



ff. skin; M.obi.int., musculus obliquus internus; >(.i., intercostal nerve; Per., peritoneum;V.p., vena parietalis. 



After Braus, 1898, Text-fig. 3. 



