102 PL ATT. [Vol. V. 



in relation to the lateral portions of the premandibular cavity, 

 suggesting that the arteries and cavities thus in their position 

 similarly related may be associated segmentally. 



5. The number of primary commissures connecting the bran- 

 chial veins in each arch is not limited to two, as seems from 

 Dohrn's (No. 6) description to be the case in Pristiurus. In 

 Acanthias I have counted as many as five commissures in each 

 of the anterior branchial arches, and the partial division of many 

 of these commissures indicates that their original number may 

 be even greater. 



6. The first aortic arch, i.e. the mandibular, unites ventrally 

 with both the hyoid vein and the anterior vein of the first 

 branchial arch. This union gives a connecting link in the hyoid 

 circulation of Selachian, Teleost, and Batrachian. 



7. The walls of the premandibular cavity give rise to the 

 superior, inferior, and internal recti muscles, and to the inferior 

 oblique muscle. The walls of an anterior prolongation of the 

 mandibular cavity give rise to the superior oblique muscle, and 

 from the walls of the so-called third head cavity is formed 

 the external rectus muscle. These observations merely confirm 

 those of Van Wijhe (No. 16), whose study of the development of 

 the eye-muscles in the Selachians did not include those of 

 Acanthias. 



8. The rudiment of a large muscle is found in the walls of 

 the mandibular cavity, in close proximity to the external rectus 

 muscle. This rudimentary muscle, like that which arises from 

 the walls of the anterior head cavity, is later lost in indifferent 

 mesoderm. 



9. My study of successive stages in the development of the 

 eye-muscles shows that the muscle cells of the external rectus 

 first appear in the median wall of the third head cavity. The 

 cells pass from this wall into the cavity, ultimately filling it. 

 This is also the case with the rudimentary muscle formed from 

 the walls of the anterior head cavity. The superior oblique 

 muscle and the rudimentary muscle of the mandibular cavity 

 appear first in the median wall of this cavity. The premandib- 

 ular cavity is formed by the fusion of a pair of lateral cavities 

 with a median space, which is generally supposed to have a mor- 

 phological value widely different from that of the two cavities 

 which it unites. If the median walls of the originally paired cavi- 



