282 J. T. CUNNINGHAM. 



these folds are a pair of muscles^ called by Schneider, in his 

 ' Beitrage zur vergl. Anatomic der Wirbelthiere/ the anal fin 

 muscles. According to Dohrn, these muscles serve to pro- 

 trude the so-called penis of the male Lamprey. Dohrn raises 

 the question of the possibility of copulation in the Lamprey, a 

 possibility which does not really exist, for in the female there 

 is a protrusible tube at the abdominal pore, which is shorter 

 but otherwise exactly similar to that of the male. Dohrn 

 suggests that the anal fin muscles of Schneider are homologous 

 with the muscles of the pelvic fin in other fishes (Selachians 

 especially). 



Origin of the Vertebrate Paired Eyes. 



The most recent study deals with the embryology and phy- 

 logeny of the Vertebrate eye. It was obvious to previous 

 embryologists that the nervous part of the eye was originally 

 in the wall of the brain. Lankester suggested that the an- 

 cestor was at this time transparent, while Balfour believed that 

 though the tissues may have been transparent, the original 

 cause of the outgrowth of the optic vesicle was the covering of 

 the original superficial eye by the formation of the medullary 

 tube. But the starting-point of Dohrn's inquiry is the deve- 

 lopment of the eye-muscles. Balfour indicated briefly the 

 origin of these muscles from the most anterior head-cavity. 

 Marshall (this Journal, vol. xxi) ascertained that only the 

 rectus internus superior, inferior, and obliquus inferior arose 

 from the prse-mandibular cavity, while the obliquus superior 

 arose from the mandibular, the rectus externus from the 

 hyoid. But Marshall believed that the dorsal parts of the 

 head cavities from which the eye-muscles were formed were 

 homologous with myotomes, and not with the ventral coelom 

 in the trunk. Dohrn does not agree with this, and holds that 

 the dorsal parts, like the ventral, are not homologous with the 

 myotomes in the trunk, but only with the ventral walls of the 

 body cavity. As a consequence of this it follows that the eye- 

 muscles are true muscles of visceral arches, and must have 

 been originally branchial muscles. The reason why branchial 



