192 bulletin: museum of compaeative zoology. 



ventral to the dorsal aorta.^ The vessel which Dohrn ('88) called the 

 carotis, aud which he stated lay ventral to the connecting stalk of the 

 first cavity, if comparable at all, is comparable only to the carotis ven- 

 tralis of Amniota.^ 



The chief arguments concerning the nature of the 2d (mandibular) 

 cavity have already been given in connection with the general question 

 of the pre-otic mesodermal segments, and it is therefore not necessary to 

 repeat them here. The evidence of a continuous lumen between this 

 cavity and the alimentary canal stated by Miss Piatt ('9P) has been 

 interpreted by her as favoring the view that the cavity is formed as an 

 ovitgrowth from the dorsal wall of the alimentary canal, similar to the 

 mesodermal pouches in Amphioxus. Kupflfer ('94), however, regards it 

 as evidence in favor of his view, that these cavities are abortive visceral 

 pouches. It is necessary, finally, to recapitulate a point in evidence 

 which has only an indirect bearing on the question of the somatic value 

 of the 1st, 2d, and 3d cavities, but which concerns vitally the morphol- 

 ogy of the eye muscles (derived in Selachii from these cavities). It 

 has been stated by Hatschek ('92) and Kupffer ('92-96) for Ammo- 

 coetes (Petromyzon Planeri). Their results tend to show that the eye 

 muscles of that low Vertebrate are, with the possible exception of the 

 muse, rectus posterior (externus), derived from splanchnic and not from 

 somatic mesoderm. According to Hatschek ('92), the muse, obliquus 

 superior appears as a differentiated portion of the muscles of the velum, 

 which correspond with the muse, adductores mandibulae. His evidence 

 (pp. 149, 150) is as follows: " Vom vorderen inneren Rande dieses Mus- 

 kels [velar muscle] dringt namlich ein Muskelfaserbiindel dorsal in das 

 Bindegewebe ein und zieht seitlich am Trabekel vorbei zwischen dem 

 ersten und zweiten Trigeminusganglion hindiu'ch bis in die Nahe des 

 Auges, wo es im Bindegewebe zugespitzt endet. Von da beginnt — 

 wie ein zweiter Muskelbauch — mit seinem zngespitzten hinteren Ende 

 der muse, obliquus superior und zieht, wieder anschwellend, in gleicher 

 Richtung weiter zum Auge. Die histologische Uebereinstimmung beider 



1 KupfEer's statement applies to that cavity in Ammocoetes which has been 

 homologized, in my opinion correctly, by most investigators (Balfour, Dohrn, 

 Shipley '87, Kupffer) with the premandibular cavity of Selachii. 



2 In my opinion those writers who have quoted Balfour ('81) and Marshall ('81) 

 as holding that the connecting stalk of the premandibular (1st) cavity is ventral 

 have misunderstood them. They both spoke of the two lateral parts of this cavity 

 as prolonged ventralwards to meet below the base of the forebrain. They give no 

 proof that the stalk is morphologically ventral, and in my opinion speak of it as 

 ventral only with reference to the wall of the brain. 



