ORIGI N AND STRUCTURE OF THE HEAD 129 



nature of the entoderm pockets. I differ from VAN Wyhe, 

 however, in that I 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 

 mouth 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 tne old stomodaeum had changed its function, the 

 first pair of gill-slits in Craniates fused to form a new 

 mouth, the second pair being 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 fiont of it; in both cases the origin lies under the mandi- 

 bular segment. I 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 the 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 

 (cf. p. 35). 



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, 

 facialis, 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 are so divergent that I shall not treat 

 them here at length but refer the reader to the short review 

 given by FuRBRINGER (1897, p. 637). VAN WVHE (1894), 

 who believes be can find the nine head segments distinguished 

 by him in Eiasmobranchs again as the first nine pairs of 

 somites of Amphioxus, tries to homologize the nerves of 

 this region to the cranial nerves of the former. 



