No. I.] THE PRIMITIVE VERTEBRATE BRAIN. 3^ 



Dr. Hoffmann, of Leiden, published in the Zoologischer An- 

 zeigcr, June 24, 1889, an article on the segmentation of the hind- 

 brain in the reptiles, which appeared after my abstract of June 

 14th had been sent to the same journal. (See Bibliography.) 

 He refers to his previous article in Bronn's "Reptilien" (pub- 

 lished in 1888), p. 1967, where he considered the hind-brain as 

 consisting of seven metameres or segments, each of which is 

 connected with a nerve in substantially the same manner as 

 described by Orr in his "Embryology of the Lizard," 1887. 

 In his more recent article, as I understand him, he considered 

 the IV. nerve to originate from the first segment of the hind- 

 brain, and to gradually shift its position forward into the mid- 

 brain. I will show that Dr. Hoffmann is probably wrong in 

 considering the hind-brain as consisting of seven segments, and 

 that the segment considered by him as the first segment of the 

 hind-brain is rather the posterior segment of the mid-brain ; in 

 other words, it is the second neuromere of the mid-brain (my 

 neuromere Trochlear, Nm. IV.). 



In addition to the above statement. Dr. Hoffmann gives the 

 following important evidence in connection with the Trochlear 

 nerve, which I quote in full. " Aus alledem scheint also mit 

 Bestimmtheit hervorzugehen, dass der N. trochlearis einen 

 dorsalen Kopfnerven bildet, denn er besitzt bei Embryonen 

 von Lacerta in jungen Entwicklungsstadien ein ziemlich mach- 

 tiges Ganglion, welches einen bis unmittelbar an die Epidermis 

 tretenden Fortsatz abgiebt, der aber, wie das Ganglion, bald 

 wieder vollstandig abortirt, ja es fragt sich selbst. Ob der Ner- 

 vus trochlearis vielleicht nicht als der vorderste, segmentale 

 Kopfnerv zu betrachten ist, der dem i, vordersten Segment 

 zuge hort : fiir diese Meinung spricht auch die Thatsache, dass 

 Ganglion, sobald es sichtbar zu werden anfangt, fast vollstan- 

 dig allein dem i. Segment aufsitzt, und spater auch auf 

 das Mittelhirn ubergreift." 



From an examination of longitudinal horizontal sections of 

 Amblystoma, Anolis, and chick embryos, the latter ranging from 

 30 hours to five days old, I find that the lateral walls of the Mye- 

 lon and Encephalon (hind, mid, and primitive fore-brain) consist 

 of a series of constrictions which are exactly alike on each side of 



