ORIGIN AND STRUCTURE OF THE HEAD 125 



form in which the first transformation has been accom- 

 plished while the latter has not yet occurred. In this 

 form we ought to find no brain vesicle, at least no praechordal 

 part of the brain; the neuropore, corresponding to the 

 mouth of Annelids, ought to be found right over the ante- 

 rior end of the notochord. just in front of the fiist pair of 

 somites which correspond to the peristomium of Annelids. 

 This now is exactly what is found originally in Amphioxus, 

 as figs. 5 and 6, after Hatschek, teach us. In front of the 

 first pair of somites there is a well-developed prae-oral lobe, 

 and the place of the neuropore, at the limit of this praeoial 

 lobe and the first pair of somites, corresponds exactly to 

 that of the mouth of Annelids. It does not correspond, how- 

 ever, to the neuropore of Craniates but to the isthmus of 

 the latter, the "provisional neuropore" of certain Vertebrates 

 in which the medullary tube has already closed when the 

 cerebral plate is still open. This provisional neuropore, 

 the future isthmus, in the same way lies right over the end 

 of the notochord and over the first pair of somites on both 

 sides of the latter. On the other hand this disposition 

 leaves no room for any other view than that the first pair of 

 mesodermic segments of fig. 5 is indeed the first and not 

 the second pair as assumed by HATSCHEK ^), VAN Wyhe and 

 WiLLEY, and that accordingly neither the "head prolonga- 

 tions" (Kopffortsatze) nor the praeoral cavities represent meso- 

 dermic segments. The spinal nerves, restricted in Craniates 

 to the epichordal part of the brain, in Amphioxus reach for- 

 ward just as far as the somites. Only secondarily does the end 

 of the notochord grow out into the prostomium, evidently 

 to provide a support for the snout which is used by the 

 animal for diving into the sand. In the same way the first 

 pair of somites sends out a prolongation into the prosto- 

 mium which, just as in Annelids, does not itself contain a 

 division of the coelom. 1 feel inclined to compare these 

 prolongations with the so called praemandibular somites in 

 Craniates. Then VAN WVHE's mandibular somites, MAC 

 Bride's collar-cavities, being the first pair of fig. 5, would 

 represent the first somatic segment. The homologization of 



') In a note Hatschek (1892, p. 137) remarks: "Wollte man den 

 rostralen Fortsatz als einen Teil des ersten Ursegmentes betrachten, 

 so wtirden sich Anderungen in der Darstellung ergeben, die jeder 

 ieicht durchfiihren kann". 



