59 
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, cerrespending to the 
mouth of Annelids, ought to be found right over the ante- 
rior end of the notochord, just in front of the first 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 vs. 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 praeo1al 
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” (Kopffortsätze) 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 WYHE's mandibular somites, MAC 
BRIDE’s collar-cavities, being the first pair of fig. 5, would 
represent the first somatic segment. The homologization of 
1) In a note HATSCHEK (1892, p. 137) remarks: “Wollte man den 
rostralen Fortsatz als einen Teil des ersten Ursegmentes betrachten, 
so würden sich Anderungen in der Darstellung ergeben, die jeder 
leicht durchführen kann”. 
