Johnston, Morphology of the Head. 199 



the brain in Petromyzon in the infundibular region some dis- 

 tance behind the future chiasma. In some other lower verte- 

 brates the same reasoning would place it in front of the chiasma. 

 The difference in the two cases is found in the length of the prae- 

 oral entoderm. Further, it appears very probable that not all 

 vertebrates agree in the position of the neuropore and in its 

 mode of closing. In mammals (36) the lower border of the 

 neuropore at the time when the eye vesicles are forming lies at 

 a point which in the adult is behind the chiasma on the topo- 

 graphically ventral wall of the brain. The point in question, 

 too, meets the entoderm and is met by the hypophysis in proc- 

 ess of formation. If this be the anterior end of the brain then 

 the optic vesicle and the olfactory bulb are dorsal organs lying 

 in succession in the order named caudal to the anterior end of 

 the brain. Since the lower border of the neuropore in lower 

 vertebrates is topographically some distance in front of the 

 chiasma, it is evident that in mammals the chiasma must form 

 across a closed up portion of the neuropore which is wanting 

 in the lower forms. Although His did, not deal with this ques- 

 tion, a re-examination of his writings with this point in mind 

 leads me to think that his observations were in accord with the 

 statement just made. In his paper on the general morphology 

 of the brain (53) he defines the anterior end of the brain as fol- 

 lows: "Diese basilare Axe [die Mittellinie des Hirnbodens] en- 

 digt unzweifelhaft in der Basilarleiste. Versteht man dagen 

 unter Gehirnaxe eine Linie, welche der Mitte der Rohrenlichtung 

 folgt, so wird diese mittlere Axe in einer Ebene liegen, 

 welche die Grund- und die Fliigelplatte des Gehirns von 

 einander scheidet, und ihr Endpunkt trifft die vordere Endflache 

 im Recessus opticus, bez. dicht vor dem Ort des chiasma 

 opticum. Wollen wir zur basilaren und zur mittleren Axe noch 

 eine dritte dorsale Langsaxe oder Langslinie annehmen, 

 so haben wir deren Ende am oberen Rande der Lamina termin- 

 alis zu suchen, vor der Stelle, wo die Fissura chorioidea ihren 

 Anfang nimmt." Since the Basilarleiste is situated behind the 

 optic chiasma it would seem that His referred the ventral 

 border of the neuropore to a point topographically ventral and 



