INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 851 



eosin, or a weak solution of hfematoxyliu ; aud, in some cases, with 

 chloride of gold. 



The cerebellum of the Vertebrata is ordinarily regarded as con- 

 sisting of four layers — fibrous, nuclear, cellular, and molecular. In 

 Petromi/zon, however, only three layers were found, inasmuch as the 

 fibrous layer is not distinctly developed. Lining the fourth ventricle 

 is an investment of epithelial cells, which have the general characters 

 of cylinder-epithelium ; their free surface is flattened, while their 

 sides form a cone which projects some way inwards, but does not seem 

 to be connected with the nervous elements ; towards the upper 

 surface, these epithelial cells become flattened and altogether lose 

 their characteristic form. Below the epithelium there is a layer of 

 nerve-fibres with small rounded cells, which are so closely packed 

 that the fibres are with difficulty detected ; it is in consequence of 

 this arrangement that it is impossible to assert the presence of a 

 fibrous, distinct from the nucleated, layer. The nerve-fibres ordi- 

 narily run parallel to the surface, and a few only pass into the mass 

 of the cerebellum to become connected with the processes of the 

 deeper lying cells. The author does not find himself able to support 

 the view of Golgi that the fibres come into connection with the 

 peripheral ganglionic cells. Three sets of fibres were distinguished : 

 (1) medullary nerve-fibres ; (2) non-medullated fibres ; and (3) fibres, 

 in which the axis-cylinder was only partially covered by medullary 

 substance. 



The nuclear-substance is composed of cells, which are rounded in 

 form, and have in the centre a highly refractive portion ; the view of 

 V. Baer, that they are arranged in groups rather than, as Denissenko 

 states, in regular rows, is stated to be the more correct ; the so-called 

 " nuclei " give off fine processes, which enter into various connections ; 

 but much of what follows is concerned with a criticism of Denissenko's 

 statements, into which it is impossible for us to enter. In the midst 

 of the cells there are to be found a number of nerve-fibres, which 

 surround the cells and are partly connected with thera by processes, 

 and the connection is so close that the author is inclined to speak of 

 a nucleo-fibrous layer. 



As to the cellular layer, the only differences between Petromyzon 

 and the frog lie in quantitative characters ; there is a series of large 

 pyriform cells — the cells of Purkinje, which are provided with two 

 processes, and have their greater portion occupied by a large nucleus ; 

 these cells are invested by a membrane of a character somewhat 

 difficult to distinguish in its continuations on to the processes of the 

 cell ; the plasma surrounding the nucleus is traversed by primitive 

 fibrillfe, which are collected together at the process into which they pass. 



The " molecular layer " is almost completely made up of the 

 branches of the peripheral processes of the cells of Purkinje, and 

 these cells though they vary greatly in direction never pass into the 

 nerve-fibres. 



Spinal Ganglia and Dorsal Medulla of Petromyzon.* — An ac- 

 count of his examination of these structures is given by Herr Freud. 

 * 'SB. Akad. Witu,' Ixxviii. (1879) p. 81. 



3 L 2 



