This conclusion is not only not at variance with the opinions of morphologists, but gives a straightforward, simple explanation why the palæostoma was situated in the very place where they are most inclined to locate it. Thus, if we trace the history of the question, {318}we see that Dohrn's original view of the comparison of the vertebrate and the annelid led him to the conception that the vertebrate mouth was formed by the coalescence of a pair of gill-slits, and that the original mouth was situated somewhere on the dorsal surface and opened into the gut by way of the infundibulum and the tube of the hypophysis. This, also, was Cunningham's view as far as the tube of the hypophysis was concerned. Beard, in 1888, holding the view that the vertebrates were derived from annelids which had lost their supra-oesophageal ganglia, and that, therefore, there was no question of an oesophageal tube piercing the central nervous system of the vertebrate, explained the close connection of the infundibulum with the hypophysis by the comparison of the tube of the hypophysis with the annelidan mouth, so that the infundibular or so-called nervous portion was a special nervous innervation for the original throat, just as Kleinenberg had shown to be the case in many annelids. Beard therefore called this opening of the hypophysial tube the old mouth, or palæostoma. Recently, in 1893, Kupffer has also put forward the view that the hypophysial opening is the palæostoma. basing this view largely upon his observations on Ammocoetes and Acipenser. [Illustration: FIG. 125.--DIAGRAM TO SHOW THE MEETING OF THE FOUR TUBES IN SUCH A VERTEBRATE AS THE LAMPREY. _Nc._, neural canal with its infundibular termination; _Nch._, notochord; _Al._, alimentary canal with its anterior diverticulum; _Hy._, hypophysial or nasal tube; _Or._, oral chamber closed by septum.] As is seen in Fig. 125, the position of this palæostoma is a very suggestive one. At this single point in Ammocoetes, four separate tubes terminate; here is the end of the notochordal tube, the termination of the infundibulum, the blind end of the nasal tube or tube {319}of the hypophysis, and the pre-oral elongation of the alimentary canal. It is perfectly simple and easy for the olfactory tube to open into any one of the other three. By opening into the infundibulum it reproduces the condition of affairs seen in the scorpion; by opening into the gut it produces the actual condition of things seen in Myxine and other vertebrates; by opening into the notochordal tube it would produce a transitional condition between the other two.