Smallwood and Rogers, Molhiscan Nerve Cells. 75 



a centrosome is present. The centrosome, when present, as well as each of the 

 inferior centers, is generally surrounded by a small clear space. 



The structure which Lenhossek designated as a centrosome received its cor- 

 rect interpretation only when the toad was studied during hibernation. Levi 

 ('98) in describing the changes in the nerve cells during hibernation gives a minute 

 account of the so-called concentric figure or vortex as it occurs in the toad. During 

 hibernation the deeply staining granular substance does not appear and the other 

 parts appear more clearly. The centrosome is nothing more than a transverse 

 section of the axis of the vortex which is composed of fibrils. These results of 

 Levi throw serious doubts on the correctness of other observations which were pub- 

 lished soon after Lenhossek's. Furthermore, w-e do not believe in the light of all 

 that has been recently discovered in the cytoplasm of nerve cells that the structures 

 described by McClure, Hamaker, and Koster are centrosomes at all, but prob- 

 ably belong to one of the classes of granules. The fewness of the radiations in the 

 results of Lewis and Rand is of itself enough to suggest a reasonable doubt as to 

 their actual presence, while the secondary and tertiary systems of radiations as 

 figured and described by Rand are not in harmony with the ordinary aster struc- 

 ture. That the centrosome is not usually found in adult nerve cells is abundantly 

 shown by numerous investigations; that it does appear in some nerve cells cannot 

 be doubted, as Hatai ('01) has shown in the young rat. The centrosome is more 

 easily seen in the young nerve cell than in the adult, which he believes indicates 

 a slight tendency to the degeneration of this structure. Most of the results referred 

 to above are so questionable that we are inclined to believe that there is very little 

 positive evidence in favor of the centrosome in adult nerve cells. 



SUMMARY. 



1. The nervous system of gasteropods does not permit of direct 

 stimulation of a specific ganglion because of the compactness of the 

 nerve collar and the numerous nerves arising from the different 

 ganglia. 



2. Lymph canals are not identical with the cytoplasmic vacu- 

 oles. They really exist, and have a rather general distribution 

 among the nerve cells of invertebrates. 



3. Vacuoles are present in the cytoplasm of nerve cells of 

 Nemerteans, Annelida, Crustacea, Insecta, and Mollusca. The 

 vacuoles can easily be seen in the living cells as independent struc- 

 tures filled with a fluid or differential bodies. They are transi- 

 tory structures, vary in number and" are not limited by distinct 

 walls. 



4. NissL bodies exist in invertebrate as well as vertebrate nerve 

 cells. They are found to occupy a zone of cytoplasm next to the 

 nucleus but not extending out to the cell wall in most instances. 

 They are chiefly arranged in rows or in spindle-shaped groups. 



