446 H. V. NEAL 



I regret that my own observations are not sufficiently extended 

 to enable me to confirm Koltzoff's statement, but I know of no 

 reason for doubting his conclusions. If they are correct, Pet- 

 ronyzon resembles an Acraniote in having all of its myotomes 

 persist in the adult. In all other Craniotes at least some myo- 

 tomes in the ear region degenerate in embryonic stages. Kolt- 

 zoff's discovery consequently now puts us in a position to com- 

 pare a larval Craniote with a larval Amphioxus and thus to carry 

 the history of the eye-muscle back to an Amphioxus stage, 

 that is, to a stage before eyes were differentiated. Here, however, 

 we are confronted with the difficulty of exact homology between 

 the myotomes of Amphioxus and those of Craniotes. 



The absence in Amphioxus of eyes, ears and serial brain vesicles 

 deprives the morphologist of the accustomed fixed points of 

 comparison. The considerable divergence of opinion regarding 

 the metameric homologies of Amphioxus is, therefore, not sur- 

 prising. The most reasonable conjecture appears to the writer 

 to be the assumption that the first permanent myotome (Van- 

 Wijhe's 1st) of Craniotes is exactly homologous with the first 

 permanent myotome of Amphioxus. This supposition is strength- 

 ened by the relations to the 'anterior cavities.' For, in both 

 Amphioxus and Craniote embryos, are found mesodermic mass- 

 es or paired cavities anterior to the first permanent myotomes. 

 These are the 'anterior entodermic diverticula' of Amphioxus and 

 the 'anterior head-cavities' of Craniotes (Elasmobranchs and 

 Ganoids) . The exact homology of these mesodermic cavities is 

 based, not only on their relation to the first permanent myotomes, 

 but also on the important circumstance that in Ganoids (Amia) the 

 anterior head-cavities open to the exterior by an external opening, 

 just as do the anterior entodermic diverticula of Amphioxus, in 

 which the left cavity opens to form the pre-oral pit. In Amia 



Figs. 14, 15, 16 Diagrams of acraniote stages of cyclostome and elasmobranch 

 embryos in comparison with a larval Amphioxus. All three show an homolo- 

 gous mesodermic segmentation. 1, 2, 3, If., etc., somites 1, 2, 3, 4, etc; a, anterior 

 cavities ;c.s.g., club-shaped gland ;enrf, endostyle;^.s.°, first (transient) gill-pouch; 

 ■g.s.' first (permanent) gill-pouch; hyp. hypophysis; L, lens; M, mouth; 'N , nasal 

 pit; n\-h, notochord; w'/j, neuropore; ol, otic capsule; sp. spiracle; i/iV, thyreoid. 



