Johnston, Forchrain Vesicle in Vertebrates. 507 



has the same rchitions but the cavity becomes obliterated. During 

 early development the evagination appears in identical manner and 

 relations in all vertebrates and the writer can see no ground for 

 doubting the complete homology of the structure in all vertebrates. 



This homology was understood by His but seems not to be univer- 

 sally accepted. Edinger (1908, p. 203) publishes a schematic figure 

 of a sagittal section of the vertebrate brain in which he shows an 

 infundibulum in contact with the hypophysis and behind it a wholly 

 separate evagination of the brain floor which he calls the saccus 

 vasculosus. This diagram stands in contradiction to the drawings 

 from actual specimens in the same book (Fig. 167, Chimsera; Fig. 

 175, 176, Varanus; Fig. 178, Ammoocetes; Fig. 181, Siredon; Fig. 

 219, Hexanchus). In all of these there is only one evagination of 

 the brain floor between the optic chiasma and the mammillary bodies 

 and it comes into relation with the hypophysis. The writer does not 

 know of any vertebrate in which the condition shown in Edinger's 

 diagram is found. 



3. Segmentation of the Neural Tube in Front of the Cerebellum. — 

 In the hindbrain the neuromeres are generally recognized as brain 

 segments corresponding to the segments of the organs in the head. 

 In front of the cerebellum there is no such unanimity of opinion. The 

 wi-iter has discussed this subject at length (1905) and has found 

 nothing in the studies here reported to change any of the conclusions 

 there expressed. On the contrary, the conclusions there based on indi- 

 rect evidence from other authors are confirmed by direct observation. 

 The segments in the mes-, di- and telencephalon are clearly indicated 

 in Fig. 42, representing parasagittal sections of the brain of a pig 

 of 7 mm. The optic vesicle is here seen somewhat out of line with 

 the other neuromeres but no one would doubt that it represents one 

 brain segment. In front of it is the first segment, from which the 

 telencephalon is formed. Opposite the optic vesicles in the median 

 region is the velum transversum. Behind the optic vesicle are 

 clearly seen in the figure three segments. In connection with the 

 first of these (neuromere iii) appears later the epiphysis. The other 

 two (iv and v) obviously enter into the mesencephalon. In selach- 

 ians these two segments have connected with them respectively the 



