Mabch 12, 1897.] 



SCIENCE. 



431 



ment only to increase the number of the 

 epithelial components or their products 

 of metamorphosis, the ependymal cells. 

 By the continuous proliferating activity of 

 the ' Keimzellen ' a considerable number of 

 ependymal cells (at least in the case of 

 higher Vertebrates) are gradually created. 

 Thus a definite framework is brought into 

 existence, in the meshes of which further 

 processes of cellular development take place 

 on prescribed lines. About this time the 

 most important diiferentations in the neural 

 tube begin. The descendants of the ' Keim- 

 zellen ' ceasing gradually to turn into epen- 

 dymal cells are transformed into the 

 mother cells of future nerve cells which, 

 provided with certain histological char- 

 acteristics, are expressively named ' neuro- 

 blasts.' In the highest Vertebrates, more- 

 over, the offspring of the ^ Keimzellen^ appear, 

 provided with still higher capacity of differ- 

 entiation, in so far as they produce a genera- 

 tion of ^indifferent cells,' which later on dif- 

 ferentiate into either nerve or neuroglia cells. 



2. The ependymal cells, as a whole, are to 

 be considered as a phylogenetically older or an 

 embryonic stage of supporting tissue which, in 

 the ascending series of the Vertebrates or in 

 the progress of ontogenetical development, 

 loses gradually its morphological and physi- 

 ological importance, and is at last replaced 

 by a canogenetio form of supporting tissue, 

 the neuroglia proper, the elements of which 

 originate, like the nerve cells, from ' indif- 

 ferent cells.' 



3. The ' indifferent cells ' have the property 

 of locomotion, (especially developed in those 

 of the cerebellum, where they give lise to 

 the formation of the superficial granular 

 layer of Obersteiner), a characteristic of the 

 formative elements of the nervous sj'stem 

 which is of great importance for a higher 

 structural complication of the latter. 



4. The so-called ' Mantelschicht ' of His is 

 in the higher Vertebrates composed of ' in- 

 different cells ' (not only of neuroblasts as 



His supposes), which later on differentiate 

 into either neuroblasts or spongioblasts (the 

 latter being the mother cells of neuroglia 

 cells) . 



5. Not all indifferent cells undergo simul- 

 taneously such an early process of differ- 

 entiation. A certain number remain for a 

 longer or shorter time in an indifferent con- 

 dition possessing moreover the property of 

 further propagation, which activity is clearly 

 shown by the appearance of karyokinetic 

 figures within the 'Mantelschicht' during 

 a certain period of development. This fur- 

 ther proliferation of the structural elements 

 of the neural tube is obviously adapted to 

 furnish the material for the later develop- 

 ment and completion of the intricate struc- 

 ture of the nervous system as it is found 

 especially in the higher Vertebrates. 



6. It is not improbable that these indiffer- 

 ent cells may play an important role in re- 

 generative processes within the central ner- 

 vous system even in postembryonic periods. 



1. Cranial Nerves of Bdellostoma dombeyi. 

 (Read by title.) 



2. The Structure of the Organ of Corti in Adul 

 Man. (Read by title.) H. Ayers. 



The Visual Centers of Arthrojwds and Verte- 

 brates. W. Patton. 



It is assumed, based on evidence advanced 

 elsewhere, that the median ocellus of Limu' 

 lus and the Arachnids is homologous with 

 the pineal eye of Vertebrates, and that 

 the lateral eyes of Limulus and the Merosto- 

 mata are homologous with the lateral eyes 

 of Vertebrates. In the Arachnids (Limtdus) , 

 and probably in Vertebrates, the distal end 

 of the median eye stalk contains one or 

 more pairs of medianly fused ocelli. (1) 

 From the proximal end of the eye stalk 

 the median eye nerves separate, and en- 

 circling the posterior part of the fore-brain, 

 just in front of the posterior commissure, 

 terminate in Limulus, on the hsemal side of 

 the fore-brain, in two great lobes which in 



