Literary Notices. cv 



It is urged that, instead of looking on the cerebellum as a distinct 

 organ which has a special function, distinct from those subserved by 

 other parts of the central nervous system, it would be more correct to 

 look on it as a part of that system, having many functions in common 

 with other parts of it, the chief difference between one part of this 

 great system and another being the degree in which different functions 

 are represented in any given part : e. g. , with regard to motor power, 

 the anterior extremity is maximally represented in the cerebrum and 

 minimally in the cerebellum, whereas the trunk muscles are minimally 

 represented in the cerebrum and maximally in the cerebellum. 



Metamerism in Vertebrates. 



Mr. Locy in a recent number of the Anatomischer Anzeiger (28 

 April, 1894,) calls attention to the fact that the division of the nerve 

 cords into segments occurs much earlier than is generally supposed. 

 He gives a careful description of the development of the nerve tube 

 in Squalus acanthias, with illustrations of the more important stages 

 in which he shows that this segmentation is one of the earliest phe- 

 nomena to appear after the embryo is outlined. The embryonic rim 

 he finds to be segmented not only before the closure of the medullary 

 tube, but even before the medullary folds have appeared. This seg- 

 mentation is primitively epiblastic, and is clearly defined throughout 

 the length of the embryo before the mesoblast has, to any extent, be- 

 come divided into somites. These segments have been traced directly 

 into those of later stages and therefore they are not evanescent. 



From a detailed study of the relations of these segments -to the 

 organs appearing later in the development of the embryo, Mr. Locy 

 draws the following conclusion : — The segmentation of the neural 

 axis is much older, historically speaking, than the division of the 

 brain into vesicles. It is a much more primitive characteristic, and 

 quite probably existed for great lapses of time before the latter arose. 

 (The order of their appearance and the time interval between the 

 two would indicate as much.) 



We thus have exhibited the relation between two distinct morpho- 

 logical processes the one, the division of the embryo into segments, and 

 the other, the modification of the head end into cranial vesicles. The 

 former is already in existence when the latter process commences, and 

 we have the cranial vesicles superimposed upon an already segmented 

 neural axis. It seems to me, therefore, that we are not justified in 

 going further than to say that there is a certain number of neural 

 segments modified to form the head, and, after various shiftings of 



