Blake, Roof of the Fomth Ventricle. 89 



sheep embryos. An investigation of a complete series is need- 

 ed in regard to this point. 



According to His the attachment of the epitheHal wall to the 

 secondary rhomboidal lip in the cerebellar region after having 

 been carried dorsad by the folding of the primary rhomboidal 

 lip, becomes shifted again by the fusion of the contiguous sur- 

 faces of the two lips. 



Stroud states that he is unable to obtain this impression 

 from his observations, but believes that it becomes shifted by a 

 process of unfolding without fusion. 



I am inclined to the view of His in the matter, namely, in 

 so far that a certain amount of fusion does take place. 



In the sections of a human embryo from which Fig. 7 was 

 drawn, the line of fusion was readily made out and is indicated 

 in the figure. However, the fusion takes place apparently for 

 only a short distance, and the further shifting is due to a greater 

 proportional growth of the ectal portion of the substance of the 

 cerebellar proton to that of the ventricular portion. 



The growth of the cerebellum in the later stages is thus 

 the opposite of that of the earlier stages, which are character- 

 ized by a greater proportional growth of the ventricular portion. 



In the same manner the tent of the cerebellum is formed 

 late in embryonic life by the growth of the ectal portions in a 

 cephalo-caudad direction, thus producing a transverse fold. 

 Fig. 26. 



It is out of the province of this paper to enter fully into 

 the discussion of the development of the cerebellum, and be- 

 sides it would be premature with the information at hand. 



But a point which has been brought out in my investiga- 

 tions I wish to record, namely, the existence in the human em- 

 bryo, at least, of a well marked ventriculus cerebelli. 



It is to be regretted that the brains from which the frontal 

 sections were made, showing this cavity, had already been cut 

 sagitally to the neighborhood of the meson. 



This cavity is well shown in Fig. 7, of an embryo of about 

 the seventh week, twenty-two millimeters N. B. length, and also 

 appears in Fig. 2, of an embryo forty-two millimeters in length. 



