NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 241 



Froriep, in speaking of neuromeres and their nerve relations, said that 

 these relations are in no way of such a kind tliat both nerves and neu- 

 romeres appear to be constituent parts of a simple organ system. The 

 nerves, especially the trigeminus and facialis, are not so constant in 

 their relation to the folds as would be expected, if the latter were 

 primary segments of the nervous system. It looks much more as if the 

 presence and position of the nerves determines the position of the folds, 

 and as if the appearance of the folds is itself a passive, mechanical one, 

 necessitated by quick growth in length in a confined space. On the 

 basis of his research, he was therefore much inclined to consider these 

 late appearing and transitory segments of the brain as morphologically 

 unimportant phenomena. 



Broman ('95) has given a somewhat extended description of the fold- 

 ings in the hindbrain of a human embryo about three weeks old. Although 

 he nowhere formulates his conclusions as to the significance of the folds, 

 it is evident that he does not regard them as of phylogenetic significance, 

 for he says that the correspondence which earlier investigators have 

 found in their relation to nerves seem to him of little help. In his 

 opinion the results diflfer too much to allow one to think that a general 

 rule prevails in the Vertebrate series as regards the number and rela- 

 tions of the foldings. The foldings, he says, are intensified in the re- 

 gions of the greatest flexure of the neural tube, and in these regions the 

 radial arrangements of cells in the foldings is also more marked. This, 

 together with the fact that the foldings are confined to the ventral half 

 of the medulla, harmonizes well with a mechanical explanation of their 

 origin. Upon the evidence that rounded cells (which he thinks are the 

 neuroblasts) with round nuclei may be distinguished in the centre of the 

 most strongly developed parts of the brain foldings, Broman ('95, p. 189) 

 forms an hypothesis concerning the origin of the separation of lateral and 

 ventral roots. He says : " "VVenn wir noch einmal alle die oben von mir 

 als Neuromeren bestimmten Falten durchmustern, finden wir, dass nur 

 das als Abduceusneuroraer bezeichnete die ventrale Wand des Hirnrohres 

 ausbaucht. Alle u\)rigen sind entweder ganz und gar davon abgedrangt, 

 oder auf dem Wege es zu werden. Dies kann natlirlicher Weise ein 

 blosser Zufall bei diesem Embryo sein." 



Since Broman's paper is, with the exception of Locy's, the most recent 

 one on the question of neuromeres, I will discuss his evidence and con- 

 clusions at some length. It is unfortunate for the purpose of discussion 

 that he has failed to identify correctly the cerebellum Anlage. What 

 he calls Cerebellumanlage is the posterior of the two secondary sub- 



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