63 
nature of the entoderm pockets. 1 differ from VAN WYHE, 
however, in that 1 do not believe that the praeoral pit 
has ever functioned as a mouth. The most probable view 
seems to me to be that in a remote ancestor where the old 
meuth and the old stomodaeum (medullary tube) still func- 
tioned, a number of gill-slits, metamerically arranged, 
arose which soon began to act as ingestion-openings. 
When tie old stomodaeum had changed its function, the 
first pair of gill-slits in Cranietes fused to form a new 
mouth, the second pair teing the spiracles. In Amphioxus, 
however, it was the left one of the second pair of gill-slits 
that developed into the secondary mouth. With this view 
the circumstance also agrees that, as pointed out by 
VAN WYHE (1907, p. 75), the thyroid gland in Craniates 
originates behind the mouth, the endostyle of Amphioxus 
in hont of it; in both cases the origin lies under the mandi- 
bular segment. 1 do not venture to explain what has caused 
this difference in the formation of the neostoma in Craniates 
and Amphioxus — and in Ascidians, as we shall see, the 
mouth is again a different structure — but it practically proves 
that the gill-slits are older than the mouth. 
We have reached the conclusion that the so-called brain 
vesicle of Amphioxus is not homologous to the brain of 
Craniates but only to the epichordal part of latter, the 
deutencephalon. In accordance with this view we do not 
find any trace of a cranial flexure, so characteristic in 
Craniates. As we have seen, the place of the animal pole 
in Craniates and Amphioxus confirms our conclusion 
CED. 221). 
_ Anterior spinal nerves. — A final question to be discussed 
is whether it is possible to recognize the four dorsal 
spinal nerves of the Craniate head, viz: the N. trigeminus, 
Jazialis, glossopharyngeus and vagus among the anterior 
spinal nerves of Amphioxus and thus to determine in 
Amphioxus the region corresponding to the head of Craniates. 
Several authors have attempted to solve this problem but 
their conclusions ate so divergent that [ shall not treat 
them here at length but refer the reader to the short review 
given by FüRBRINGER (1897, p. 637). VAN WYHE (1894). 
who believes he can find the nine head segments distinguished 
by him ín Elasmobranchs again in the first nine pairs of 
somites of Amphioxus, tries to homologize the nerves of 
this region to the cranial nerves of the former. 
