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in a convincing way by embryology, is yet rendered probable 

 by a comparison with Amphioxus. The mouth of Craniotes is at 

 any rate a different one from that of Ampiüoxus which corresponds 

 to the left spiracle (van Wuhe, 1907^)), while that of Ascidian-larvae 

 is again another one. It breaks through at the place of the neuropore 

 so that even the anterior part of the medullary tube, tiie former 

 stomodaeum, contributes towards the formation of the new entrance 

 to the gut (Huntsman, 1913). The secondary nature of the Vertebrate 

 mouth is thus clearly shown. 



While in Ampliioxus the' endostyle arises as a ventral bulging out 

 of the gut just in front of the mouth, in the first somatic segment, 

 in Ammocoetes it originates, like the rudiment of the thyroid gland 

 in Craniates, in the same segment but consequently just behind the 

 mouth (Van Wuhe, 1907, p. 75). 



From the anterior post-branchial myotomes (Neal, 1897, p. 444, 

 KoLTZOFF, 1902, p. 304) ventral buds grow out, similar to those 

 which in Gnathostomes produce the musculature of the paired limbs. 

 Growing down behind the last gill-slit and then forward they give 

 rise to the hypobranchial musculature which is supplied by the 

 ventral roots of the same post-branchial myotomes (7^^^ — 12'^ post-otic 

 somite after Neal) that have produced the muscles, as holds equally 

 for the musculature of the limbs. The original eumetamerism of 

 gill-slits and somites afterwards, as in other Craniates, gets lost, by 

 a backward extension of the branchial sac, by which the gill-slits 

 are caused to disperse and the originally post-branchial myotomes 

 7 — 12 now come to lie epibranchially. The ventral roots of these 

 somites continue to pass behind the last gill-slit on their way to the 

 hypobranchial muscles and during the elongation of the branchial 

 basket they unite one by one to a common horizontal stem, which 

 springs from six roots and bends downward behind the last gill-slit. 

 This stem we can call the hypoglossus or plexus cervicalis (fig. 1). 

 (See following page). 



The primarily epibranchial somites give rise to the epibranchial 

 musculature, supplied by the corresponding ventral roots in front 

 of the hypoglossus. 



The 5^i\ 7^'i, 9^'' and 10*^'^ cranial nerves of Craniotes innervate 

 the primordial branchial musculature (Musculi constrictores) which, 

 though striated, must be counted with the visceral musculature 



1) Regarding the mouth of Amphioxus we now come to a conclusion somewhat 

 different from that reached formerly (1913, b). An interpretation of the praeman- 

 dibular cavity of Craniotes diverging from that of van Wuhk (1882) leads us to 

 a confirmation of van Wijhe's conclusions on another subject. 



