xeal: xeeyous system in squalus acaxthias. 203 



determine what is dorsal and what is ventral, it would follow from the 

 evidence already stated by Piatt ('91) that the anterior portion of the 

 dorsal aorta iu Squalus embryos comes to lie in part dorsal to the chorda, 

 and therefore that this organ, commonly known as chorda dorsalis, could 

 more correctly be named chorda ventralis. KupfFer's argument thus 

 leads to a reductio ad absurdum. 



According to Hoffmann ('96) the muscles innervated by the oculo- 

 motorius have their origin from the posterior part (" Fortsatz ") of the 

 premandibular cavity. Because of the complicated development and 

 the secondary subdivisions of this cavity, it is difl&cult to be certain ; 

 yet it seems to me that, as in the case of the second and third cavities, 

 the epithelium of both median and lateral walls participates in the pro- 

 duction of the muscles formed from this cavity, viz. muse, obliquus 

 inferior, and recti inferior, superior, and anterior. 



Before passing to a consideration of the nature of the " anterior 

 cavities," I wish to discuss, in connection with the preceding study of 

 the morphology of the eye-muscle somites in Squalus, the evidence of 

 the development of the eye muscles of Petromyzon which has been given 

 by Hatschek ('92) and Kupffer (91), and to determine in how far this 

 brings us to an understanding of the morphology of the eye muscles in 

 Vertebrates in general. The repeated confirmation of Marshall's conclu- 

 sion that the eye muscles in Selachii and Eeptilia are derived from the 

 epithelium of the first, second, and third cavities — van Wijhe ('82), 

 Dohrn ('85), Orr ('87), Kastschenko ('88), Miss Piatt ('91), Oppel ('92), 

 Hoffmann ('96), and myself — seems sufl&cient to remove any doubt (so 

 far as those gi'oups of animals are concerned) which Kupffer ('94) may 

 have sought to throw upon that conclusion. In Amphibia, Birds, and 

 Mammals, as is well known, the eye muscles are differentiated from the 

 connective-tissue capsule surrounding the eye. Although the source of 

 these cells is not known with certainty, there is no reason to doubt that, 

 as in Selachii and Eeptilia, they have their origin from the dorsal meso- 

 derm. In direct contradiction to these facts, which hold true for higher 

 Vertebrates, stand the conclusions of Hatschek and Kupffer, that in Cy- 

 clostomes the eye muscles are splanchnic in their origin, i. e. derived from 

 the mesoderm of the visceral arches. Let us examine the evidence given 

 by them, in order to determine in how far it seems to warrant their con- 

 clusions. Hatschek's briefly summarized evidence has been stated on 

 pages 192, 193, and needs no repetition. 



In sections of a 5 cm. Ammocoetes I find the relationship of the 

 median posterior musculature of the eye capsule to the velar muscle, 



