94 Journal of Comparative Neurology, 



defined as that part of the epicoelian roof which is clothed with 

 ectocinerea. 



In its early stages it consists of two fusiform protons which 

 lie in the latero-caudal convexity of the roof of the epencephal 

 just cephalad of the kilos and are twisted so as to conform with 

 the general outline of the walls. In the stage shown in Figs, 

 6 and 7, each extends from near the floor successively cephalad, 

 dorsad, caudad, mesad, and again cephalad and ends at a notch 

 in the roof situated at about one fourth the distance from the 

 meson to the lateral border and forms the cephalic arm of the 

 U-bend. In Fig. 5, a younger embryo, the proton and adja- 

 cent parietes are folded over ecto-cephalad. Compare Figs. 7, 

 29, 32, 50, 51, 52, 53. 



The further development appears to be a process of prolif- 

 eration, by which these lateral protons are extended dorsad, 

 cephalad, and mesad. The mesal extension meets its opposite 

 at the meson and fuses therewith. From now on we have to deal 

 with a single structure instead of with two. Reference to Figs, 

 42 and 43, PI. IV, shows that the dorsal extension at the base 

 is not as wide as might be expected, but that the organ widens 

 as it extends dorsad. This is still more apparent in human em- 

 bryos. The result of this method of growth is the spreading, 

 umbrella-like appearance so characteristic of the adult organ. 



The transition from the condition shown in Figs. 4, 5, PI. 

 I, and in Figs. 52, 53, PI. V, to that shown in Figs, 8, 9, PI, 

 I, and 58, 59, PI. VI, is very surprising. The explanation which 

 seems most reasonable to me, is that during the process of 

 growth the cerebellum, still thin at the meson, and quite flexi- 

 ble, becomes rotated caudo-ventrad, or folded upon itself; and 

 in this way the ripa is brought from the position it occupies 

 in Fig. 52 to that shown in Fig. 58. It seems to me that the 

 specimens shown in Figs. 35, 36, 37, PI. Ill, in Figs. 42, 43, 

 PI, IV, and in Figs. 56, 57, PI. V, support this view. 



His ([5, 22) has a different idea. He says, " Die Ober- 

 flache des Kleinhirns verwachst mit dem sie beriihrenden Theile 

 der Deckplatte, an Stelle der primaren vor dem Cerebellum be- 

 findlichen Taenia, entsteht eine secundare vom hintern Rande 



