50 THE ANCESTRY OF VERTEBRATES 



of the prostomium incorporated into the brain, i.e. from the 

 ventral half in Annelids, while the ciliated pits belong to the 

 dorsal half and are not closed round. Thus we can only 

 assume, that the connection between the brain and the 

 olfactory pits in Vertebrates is a secondary one compared 

 to that in Annelids. 



Primary brain-axis. — A number of authors have discussed 

 the question of the primary brain axis and its anterior end 

 and have come to divergent conclusions. This brain axis 

 of course is the forward, continuation of the axis of \ht 

 meduliary tube, the anterior dilated part of which is con- 

 sidered as being represented by the brain. According to 

 V. Baer (1828), MIHALKOVICS (1877), HIS 0893, p. 98), 

 KOLTZOFF (1902, p. 553) and Hatschek (1892, p. 139, 1909, 

 p. 497) the original fore-end of the Craniate brain, corres- 

 ponding to the neuiopore of Amphioxus, is to be found in 

 the infundibulum, and the part that lies in front of the 

 latter, the praechordal part, has secondarily grov.n out over 

 it by the strong development of the originally dorsal parts. 

 This conception finds its most striking expression in VAN 

 Wyhe's (1882, p. 39) view that the nervus olfactorius in 

 reality is the second, the nervus opticus the first nerve of 

 the head. V. KUPFFER (1894, 19i.6), on the contrary, starts 

 from the idea, that, to find the fore-end of the brain-axis 

 we must look for the neuropore of Craniates, which in his 

 opinion is directly comparable with the neuropore of y4/np/z/o- 

 xus (cf. p. 38). He finds the latter in the sturgeon as a point 

 where the brainwall remains fused with the epiderm of the 

 head for a somewhat longer time, the angulus terminalis (HIS) 

 or recessus neuroporicus (cf. fig. 13), situated more dorsally, 

 just in front of the /a/n//2a /er/72//2a//s or front-wall of the brain. 

 It indicates at the same time the anterior end of the medullary 

 suture, just as in Amphioxus, and the front-wall of the 

 brain has been formed by the growing up of the transverse 

 brain fold of the cerebral plate. To this conception HIS 

 objects, that, in transverse sections, the medullary suture may 

 be shown to continue in front of the angulus terminalis as 

 a frontal suture, probably reaching to a point where in 

 other forms, e. g. in Petromyzon, a much more terminally 

 situated neuropore, or better: an indication of the latter, 

 is found. According to HIS and HATSCHEK both the 

 "angular" and the "terminal" neuropore represent the 

 extremities of an original slit-like neuropore, as observed by 



