[159] Löwe (No. [341]) holds that at an early stage of development three regions can always be distinguished in any section of the central canal, viz. (1) a ventral narrow slit, (2) a median enlargement, and (3) a dorsal slit. Such a form can no doubt often be observed, but my own observations do not lead me to attach any special importance to it.
[160] This holds true at first for Elasmobranchii, but at a later stage there are present numerous nerve-cells in the white matter, so that the distinction between the white and grey matter becomes much less marked than in higher types; in this respect Elasmobranchii present an approximation to Amphioxus.
[161] It is not within the scope of this work to give an account of the histogenesis of the brain; in the statement in the text only a few points, of some morphological importance, are touched on.
[162] I have worked out these changes in Elasmobranchii, Amphibia (Salamandra) and Aves.
[163] “Ueb. d. Bau d. centralen Nervensystems d. Axolotl.” Zeit. f. wiss. Zool., Vol. XXV. 1875.
[164] For the relations of these bodies, vide L. Stieda, “Stud. üb. d. centrale Nervensystem d. Knochenfische.” Zeit. f. wiss. Zool. Vol. XVIII. 1868.
[165] For a full account of this subject vide Ehlers (No. [337]).
[166] Scott states that in the larva of Petromyzon the pituitary body is derived from the walls of the nasal pit; Quart. J. of Micr. Science, Vol. XXI. p. 750. I have not myself completely followed its development in Petromyzon, but I have observed a slight diverticulum of the stomodæum which I believe gives origin to it. Fuller details are in any case required before we can admit so great a divergence from the normal development as is indicated by Scott’s statements.
[167] “Les Ascidies simples des Côtes de France.” Archives de Biologie expér. et générale, Vol. III. 1874, p. 329.
[168] A comparison of the mode of development of this septum with that of the septum lucidum with its contained commissures in Mammalia clearly shews that the two structures are not homologous, and that Miklucho-Maclay is in error in attempting to treat them as being so.