24 WILLIAM MORTON WHEELER, 



The aortic cavity leads on the right, and for a short distance 

 on the left into a space between the pronephric tubules and the 



gation by Stöhr (1896), Bergfeldt (1896), Franz (1897) and Klaatsch 

 (1897). The last author attempts to show that it is the homologue of 

 the epipharyngeal groove of Amphioxus, an ingenious and, at first 

 sight, plausible hypothesis. It seems to me, however, that certain con- 

 ditions in Cyclostomes cast some doubt on Kt.aatsch's view. Petro- 

 myzon has a well-developed hypochorda, as may be seen from a cur- 

 sory glance at the figures of v. Kupfper (1890) and Goette (1890), and 

 from those accompanying the present paper. This structure soon dis- 

 appears (in my stage 5) and I am quite certain that it neither gives rise 

 to a ligament, nor to any portion of the aortic wall, nor, indeed, to any 

 specific structure ; on the contrary, the cells of which it is composed 

 become indistinguishable from the sclerotome cells in the vicinity. In 

 the Ammocœtes the mid-dorsal wall of the pharynx is pushed down 

 into the branchial cavity in such a way as to suggest a comparison 

 with the dorsal lamina of Tunicates. The structure of this region of 

 the pharynx has been described by Schneider (1879) and more recently 

 and more accurately by Schaefer (1895). This dorsal lamina consists 

 of a fold of the branchial entoderm enclosing a solid, ridge-like pro- 

 jection of the periaortic mesoderm. In the young Ammocœtes there 

 is thus obviously nothing that can be homologized with the epibranchial 

 groove of Amphioxus and Klaatsch's hypothesis is so far safe, but 

 during metamorphosis the definitive oesophagus, which arises as Nestler 

 has shown (1890a and 1890b), from a solid ridge of cells growing up 

 from the mid-dorsal entoderm of the pharynx into the periaortic tissue, 

 is very suggestive of the epipharyngeal groove of Amphioxus (conf. 

 Nestler's figures: 90b, figs. 22, 23 and 31, tab. 7). The solid ridge 

 is finally separated from the pharynx, acquires a lumen of its own and 

 becomes connected with the intestine, thus leaving the pharynx with 

 a blind posterior end. I would regard the oesophagus of Petromyzon 

 as an epipharyngeal groove greatly retarded in its development, and if 

 this views should proove to be correct, Klaatsch's hypothesis would 

 have to be abandoned, since the groove and the hypochorda could not 

 coexist in the same animal on his assumption. 



Prenant (1898) has recently described an outgrowth from the 

 entodermal wall of the pharynx in certain snakes and lizards and 

 he regards it as the homologue of the hypochorda of the Ichthyopsida. 

 Prom an inspection of his figures this does not seem probable, for the 

 structure he represents is much more voluminous and persists much 

 later than the Ichthyopsid hypochorda, which is itself only an evanes- 

 cent and unstable vestigial organ. Moreover the structure in Reptiles 

 long retains its connection with the entoderm of the pharynx and does 

 not from the first adhere to the lower surface of the chorda as in Ich- 

 thyopsida. Finally, the structure described by Prenant comes to lie 

 below, whereas the true hypochorda lies above the aorta. It is, of 



