38 THE ANCESTRY OF VERTEBRATES 



into two regions is very evident. Immediately after the 

 closure of the medullary plate we can distinguish, with 

 V. KuPFFER (1905), the archencephalon and the deuteren- 

 cephalon, the former praechordal and giving rise to the 

 optic vesicles, and the latter epichordal. The two are 

 separated from each other by ihe plica ventralis, a transverse 

 fold of the ventral wall of the brain, right over the fore-end 

 of the notochord. The archencephalon is thus a well defined 

 vesicle; the deuterencephalon tapers insensibly, without a 

 distinct limit, into the medullary tube of which it is indeed 

 only the anterior dilated part. The plica ventralis indicates 

 at the same time the place of the cephalic flexure and lies 

 right over the sella turcica which, after GegENBAUR (1872, 

 p. 119), indicates the limit between the segmented "verte- 

 bral" and the unsegmented "praevertebral" part of the skull. 

 From the archencephalon the telencephalon andthe dience- 

 phalon afterwards develop, from the deuterencephalon, whose 

 cavity becomes the fourth ventricle, the myelencephalon 

 and, if present, the metencephalon. For the mesencephalon, 

 whose cavity becomes Ihe iter, it is hard to say from which 

 of the two it is to be derived and if it is to be derived 

 from either of them. It lies exactly above the plica ventralis. 

 At any rate it is in the region of the isthmus, between meso- 

 and metencephalon, that we have to look in Craniates for 

 the neuropore of Amphioxus which represents the old mouth. 

 From the above considerations it follows that the brain vesicle 

 of Amphioxus is to be compared with the deuterencephalon 

 of Craniates and not with the archencephalon, as KUPFFER 

 himself supposed. From this point of view it might appear 

 adequate to interchange the names arch- and deuterence- 

 phalon, the latter phylogenetically being present before the 

 archencepalon. I will not, however, propose a change of 

 names when introduced by such an authority as V. KuPFFER 

 and generally adopted. Not only would such a change 

 give rise to confusion but arguments may be equally 

 well adduced for the present nomenclature, for the archen- 

 cephalon, together with the eyes, develops from the same 

 source as the cerebral ganglia and eyes in worms, molluscs 

 and arthropods, viz: from the apical plate, and that their 

 homologue is absent in Amphioxus is evidently due only 

 to secondary circumstances. In this respect then the archen- 

 cephalon has older claims to the name of brain than the 

 deuterencephalon. 



