MUSEUM OF COMPARATIVE ZOOLOGY. 151 



Among the Metazoa, epithelial tissues oflPer by far the greatest num- 

 ber and the most interesting cases of amitosis. Furthermore, as Ziegler 

 ('91) has very recently shown, epithelial cells of uimsual size, with some 

 peculiar functional activity {generally secretion) are most apt to exhibit 

 this method of division. Cell division has seldom been observed to fol- 

 low amitosis in such large cells, which therefore become multinucleate. 

 Other epithelial cells which frequently furnish instances of amitosis are 

 those which are near the end of their functional activity. Cells of the outer 

 layer of a stratified epithelium sometimes divide amitotically, while 

 those of the deeper (and therefore younger) layers of the same epithelium 

 divide by mitosis. A good instance of this was recently described by 

 Dogiel ("90) in the epithelium of the bladder of Mammals. The nuclei 

 of the large epithelial cells lining the intestine of Arthropods very com- 

 monly divide by amitosis, as was found by Frenzel ('85) in the midgut of 

 Astacus and Maja ; by Carnoy ('85) in the intestinal epithelium of Iso- 

 pods; and by Faussek ('87) in the digestive tract of a Cricket (^Eremobia 

 muricata) and in the larva of uEschna. The intestinal epithelium in all 

 Arthropods has an important secretory function. Cells whose function 

 is excretory likewise exhibit amitotic division of the nucleus, as in the 

 Malpighian vessels of Insects. The occurrence of amitosis in glandular 

 and excretory epitlielium is readily explainable on Chun's hypothesis, 

 for the functional activities of such cells are peculiarly intense, and it is 

 easy to see that a distribution of nuclear material in the cytoplasm is 

 of advantage to the cell. The occurrence of nuclei of unusual size (as 

 compared with the nuclei of other cells of the same animal) seems to 

 me likewise referable to the peculiar needs of the cytoplasm in these 

 cells. 



Cases of amitosis peculiarly difficult of explanation are those pre- 

 sented by the germinal epithelium of the testis. So many observers 

 have reported direct division in sperm mother-cells, that there seems 

 no reasonable doubt of its occurrence. It has been suggested that the 

 cells which divide amitotically never produce spermatozoa, but merely 

 serve to secrete a fluid. This explanation, however, will not serve in 

 the case of certain Isopods {Oniscus asellus and Idotea sp.) in the testes 

 of which Carnoy ('85, p. 222) found amitosis the prevailing type of di- 

 vision, and mitosis of very rare occurrence. Direct division is found 

 more or less frequently in the testicular cells of many other Crustacea, 

 as the extensive woi-k of Gilson ('84-87), and the investigations of 

 Sabatier ('85) show, and occasionally in the other groups of the Arthro- 

 pods. Among Vermes, it was found by Lee ('87) in Xemertiaus, and 



