36 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the regeneration of the tail. It is hence suggested that in animals the- 

 absence of regeneration in an organ may be caused by the stoppage 

 of development of the supporting tissue due to the fibrous growth of 

 the cicatrix. 



Development of Amphibian Excretory System.*—!). P. Filatow 

 discusses the development, structure, and functions of the pronephrie 

 glomus, and the development of the mesonephric tubules. He explains 

 certain differences in the first stages of the tubules in Urodela and 

 Anura, referring particularly to the process of separating the connecting 

 group of cells from the lateral plate, and the mode of approach of the 

 group to the duct. In the Anura the group is first detached before- 

 approaching the duct, while in Urodela the process of separation is pro- 

 longed, with the result that a different formation appears. He thinks- 

 that this explanation may account for peculiarities of the tubule forma- 

 tion in other animal groups. 



Development of Ventral Nerves in Selachii.t — H. V. Neal dis- 

 cusses the spinal ventral nerves in the spiny dogfish {Squalus acanthias). 

 Positive conclusions in regard to some important questions have been 

 reached. (1) The neuraxones of the spinal ventral nerves of Selachians 

 develop like those of the Amniota as. processes of neuroblast cells. In 

 their growth they are secondarily surrounded by sheath-cells. (2) Me- 

 dullary cells, but not those that form the neuraxones, migrate into the 

 ventral nerves in the early stages of development. (3) The migrant 

 medullary cells form the neurilemma sheaths, but take no part in the 

 formation of the neuraxones or ganglia of the ventral nerves. (4) The 

 epineurium and perineurium sheaths are in chief part added to the 

 embryonic nerve from the adjacent mesenchyme. 



In his summary the author states that neuroblasts and spongioblasts 

 are undifferentiated in the early stages of the ventral nerve ; the first 

 neuraxones are formed before the migration of the cells Avhich produce 

 them ; the " germinative cells " of this are simply mitotic cells • no 

 neuroblasts migrate from the wall of the neural tube ; their migration 

 is entirely within the wall, and is the passive result of the multiplica- 

 tion of cells near the lumen of the tube ; neuraxones of spinal ventral 

 nerves are formed exclusively by medullary cells ; the cells of ventral 

 nerves are not concerned with the formation of neuraxones ; they form 

 the neurilemma and possibly also the connective-tissue sheaths of the 

 nerves, to which mesenchymatous cells probably also contribute ; the 

 cells of the forming ventral nerve are migrant medullary elements, to 

 which are subsequently added cells from the adjacent mesenchyme ; the 

 first connection of ventral nerve and myotome is not an intimate neuro- 

 muscular attachment ; there is no primary cellular connection such as 

 has been postulated by Sedgwick. 



Torus longitudinalis of Teleost Brain4 — P. E. "Sargent discusses 

 this archaic portion of the mesencephalic roof formed in the Teleosts 



* Anat. Anzeig., xxv. (1904) pp. 33-47. 



t Mark Anniv. Vol., 1903, pp. 291-31:; (3 pis.). 



; Tom. cit., pp 399-416 (1 pi.). 



