Stroud, Mammalian Cerebellum, 93 



of the roof of this segment is involved in the development of 

 the cerebellum ? 



The adult condition may here be considered. Compare 

 Fig. 24, PI. Ill with Figs. 42, 43, 44, PI. IV. In the adult 

 brain we have as mesal landmarks, 



1. The mesencephal and isthmus. 



2. The kilos. 

 Between the structures lie, 



a. The valvula. 



b. The cerebellum. 



At the constriction of the isthmus, there is a thickening in 

 the roof which increases laterad. It is in fact "a transverse 

 plate inclined at a wide angle to the axis of the oblongata." 

 See Fig. 24, PI. III. Now the universal statement in all text- 

 books on embryology is that this transverse plate is the cerebel- 

 lum. It is true that it does bear a resemblance to the amphib- 

 ian cerebellum. But if this plate is the cerebellum, ivhere is the 

 valvjcla f What also is the meaning of the two lateral thicken- 

 ings, in the caudal part of the roof, which do not extend across 

 the meson. See the section shown in Fig. 24 «, PI. Ill, and 

 also gross preparations shown in Figs. 4, 6, PI. I, 52, 54, 55, 

 PI. V. Again what is the meaning of the condition shown in 

 Figs. 8 and 10, PI. I, and Fig. 58, PI. VI, where these lateral 

 protons have fused upon the meson ? 



The only explanation which the writer, at present, has to 

 offer is this : that the thickening at the isthmus is really less 

 than it appears to be. For, suppose that as a result of the dila- 

 tation of the mesencephal its walls become thinner. The effect 

 would be to make the appearance of an isthmus thickening 

 greater than it really is. 



The writer believes that the results of his own observations 

 upon the brains of both man and the cat go to show that this 

 so-called transverse plate is not the cerebellum, and that in early 

 embryos the cerebellum does not appear in a mesal section. 



The Ccrebellmn. 



Definition. — The cerebellum, in its broadest sense, may be 



