TO JOURNAI. OK CoMP AKATIXK XkCROLOGY. 



positions, as may be gathered from sections of earlier 

 embryos than that figured on Plate II, Fig. 4. It may be 

 remarked in passing that, in our opinion, the latter analogy 

 is misleading, since, as we shall hope to show, the pro- 

 liferating centres from which the cortical cells are derived 

 are central or ental rather than superficial. The solution 

 of the problem of the histogenesis of the cerebellum is to be 

 sought in earliest stages of its development, which can be 

 shown to follow in a condensed manner the course just 

 indicated for reptiles. 



Returning now to the embryo mammal (Fig. 2, Plate III) 

 and again noticing that the dorsal surface is devoid of cells 

 except at the caudad extremity, we observe that in an older 

 embryo (Fig. 3) a much larger portion of the dorsal surface 

 has been thus covered, and evidently from behind forward. 

 Fig. I, which is a transverse section through one-half the 

 cerebellum near its base and part of the medulla in a mouse 

 embryo of a little older stage, shows that the entire surface 

 has now been covered by the proliferous zone, but that it is 

 curiously double, with a layer of white fibres separating 

 the two zones. It is also observed that the walls of the 

 nerve-tube at the recessus lateralis of the fourth ventricle 

 are very thin, and consist of very rapidly proliferating and 

 hence closely-packed cells which pass from the ental to the 

 ectal surface. This section suggests that possibly the lateral 

 (and caudad) portions of the ventricular surface of the cere- 

 bellum may be the sources of the proliferating superficial 

 layer of the dorson. Yet we have no clue to the double 

 character of this proliferous zone, and have no direct proof 

 that the suggestion is valid. Before we can secure such 

 proof it will be necessary to thoroughly understand the 

 nature of these lateral recesses of the fourth ventricle. The 

 description given by Mihalkovics, though incomplete, is very 

 suggestive: 



Bemerkenswerth von der Seitentasche [Jiecesstis lateralis'\ ist, dass 

 sie itn ausgebildeten menschlichen Gehiro aus zwei einander beriihr- 



