( 28) 



Pelroiiiyzon hu'\aü to (lie iiiid-hraiii iiurk'us of higlier verlehniles. 

 As for Aiiiinocoetes 1 liuvc no expericjice, Imt lor Petromyzon I ciiiiiiot 

 sliare this supposition. For the rest 1 cannot say anything with 

 certainty. 



All other vertebrates which I examined, from selachii lo man, 

 possess a mesenc. trigeminus nucleus and accompanying root. Neither 

 on mvelinc sheath i)reparations, nor on carmine or v. Gikson pre- 

 parations is the transition of axis-cvlinders IVom the corresponding 

 cells in the rad. mesenc. even observable. It is possible lo state this 

 fact on silver preparations treated after Cajal or Biet,s(jhowskv, of 

 which 1 convinced myself in sharks (Acanthias vulgaris, ^), birds 

 (hen embryos, ■) and mammals (rabbit and cat). 



The root then reaches the latero-dorsal, and afterwards the lateral 

 boundary of the central grey substance of the aqueductus Sylvii, runs 

 with a few exceptions laterally from the undecussated trochlear root 

 distally, till with the nerv. trigeminus it passes wholly oi', according 

 to others, partly outside the oblongata (pons) — after sending oil" 

 (in most animals) collaterals to the pjotor V nucleus. 



In the structure of the cells there are certain variations. In all 

 mammals — except marsupials and perhaps monotremata — the 

 cells without exception are on cross section round or elliptical, 

 according to the direction in which they are cut. This gives them 

 a slight resemblance to spinal gangliacells to which we have already 

 alluded. In all other species of animals this form is seen again, 

 but generally mingled with elements of a more [)olygonal shape ; 

 exceptions to this are birds, some re[)tiles and amphibians ') where 

 only round or o\'al cells are found. 



In marsujMals {(lide/ijhi/s shows it better than macro pus) both 

 kinds are found together, almost in the same way as among the 

 reptiles in chcloiu' midas. In tishes the i)olyedric cells predominate, 

 in carmine or haematoxylin preparations they frequently look like 

 heavy lumps of protoplasm. 



According to Cajal, after successful silver impregnation in some cells 

 of the brains of young mammals (cat, rat, rabbit) several, genei-ally 

 short, neurones are to be found, besides the neurones wdiich will 

 form the mesenc. root. I observed the same dendrites in a few cells 

 of acanthias, and in embryos of raja, as Held had previously done 



1) In the Anatomical cabinet at Leyden. (Prof Boeke). 



2) In the Anatomical Lal)oiolory at Groningen. (Prof, van Wijhe). 



3) According to P. Ramon. Trab. biol. Madrid. Vol. Ill, p. 153. 1 did not succeed 

 in discovering these cells in rana or salamander. 



