120 THE ANCESTRY OF VERTEBRATES 



The second pair of gill-slits would have atrophied, they 

 are to be found again in the left and the right pseudo- 

 branchial groove or "vorderer Wimperbogen" and corres- 

 pond to the spiracle of fishes. The first permanent pair of 

 gill-slits accordingly represents in reality the third pair. 

 Recently Hatschek (1909, p. 508) seems to have changed 

 his opinion and offers a prospect of a different interpretation 

 and a homologization of both the "anterior entoderm pockets" 

 and the "rostral prolongations" to the praemandibular and 

 mandibular segments, "und zwar in einer Weise, die zu der 

 herrschenden Anschauung im Gegensatze steht". Evidently 

 he (1906, p. 6) now wants to consider the head cavities as 

 the ventral part (hyposomite) of his anterior pair of somites 

 of which the head-prolongations represent the dorsal part 

 (episomite). 



Similar interpretations have been given already by VAN 

 WYHE (1893, p. 157) and WiLLEY (1894, p. 126). WiLLEY 

 compares both the "anterior entoderm pockets" (HATSCHEK) 

 or "praeoral head cavities" (WiLLEY) with the praemandibular 

 segments of Craniates and the anterior pair of somites of 

 fig. 5 with the mandibular segment of the latter. VAN 

 Wyhe agrees with him in the latter assumption but not 

 in the former. In the two anterior entoderm pockets he 

 cannot see antimers, they are in reality median structures 

 and the pocket that only secondarily, in consequence of 

 the rotatory growth of the anterior part of the alimentary 

 tract, has become the left one, and that passes into the 

 praeoral pit, represents the old mouth of Amphioxus, the 

 "autostoma", comparable to the mouth of Craniates. Accord- 

 ing to Van Wyhe the right one, passing into the praeoral 

 head coelom, may be homologous to both the praemandi- 

 bular cavities of Craniotes. As to the first pair of gill-slits 

 Van Wyhe came to very remarkable and noteworthy con- 

 clusions. His (1887, p. 429) had observed already that the 

 mouth in Amphioxus "lasst sich seiner Stellung nach eher 

 einer Kiemenspalte vergleichen. An der entsprechenden 

 Stelle der anderen Seite bildetsich die sogenannte kolben- 

 formige DrUse." VAN Wyhe (1893, p. 153) now found that 

 the musculature and the innervation show that the mouth of 

 Amphioxus is not a median structure but one that wholly 

 belongs to the left side. It is situated behind the mandi- 

 bular segment, as VAN WYHE calls the first segment of 

 fig. 5, and is therefore compared by him to the first left 



