in the Animal Kiiujdoin. 371 



or two smaller ones, or even three, four, or five ; the nuclei 

 themselves are found in all stages of direct division. 



The conditions presented bj the epithelium of the mid-gut 

 of Insects * and Crustacea require special discussion. After 

 a critical examination of the literature we must arrive at the 

 conclusion that in such epithelial cells as are already func- 

 tioning as gland-cells, or in which the process of secretion is 

 just beginning, direct nuclear division may occur ; that these 

 cells and their nuclei are then gradually or periodically cast 

 off, and that the regeneration of the epithelium usually 

 proceeds from isolated groups of young cells, or from regene- 

 ration-pits, the cells of which multiply by mitosis. Frenzel'sf 

 observations also admit of interpretation in this sense. This 

 author noticed in the intestinal epithelium of Phronima a few 

 scattered islands of younger cells, which were not engaged in 

 secretion and multiplied actively by mitosis. In Astacus^ 

 Maja^ and Dromia he observed typical amitotic nuclear divi- 

 sion \. As regards Insects, Frenzel writes as follows : — " The 

 cells of the mid-gut have to perform the task of furnishing 

 the digestive secretion, and a portion of them, namely the 

 true epithelial cells, in the caterpillars the columnar as well as 

 the mucous-cells, constantly perish in so doing " §. " The 

 true epithelial cells in the mid-gut of Insects, it matters not 

 whether they belong to the actual intestinal tube or to its 

 evagi nations, or whether they are to be ascribed to the type 

 of elongated columnar cells, or to that of rounded raucous 

 cells, propagate by the method of direct amitotic nuclear 

 division." iSo far Frenzel's statements agree very well with 



* Amitotic nuclear division occurs not only in the mid-gut, but also in 

 the hind-gut of Insects. P'aussek ( " Beitrage zur Histologie des Dcxrni- 

 kanals der Insekten,'' Zeitschr. f. wiss. Zoologie, Bd. 45, 1887} observed 

 it in the hind-gut of a grasshopper {Ereinohia miiruata, Pall.) and in the 

 rectal glands of ^sc/iwr/-larvae. So far as we know, this division of the 

 niicleiis is not followed bv a division of the cell. 



t J. Frenzel, '• Ueber den Darnikaual der Ciiistaceen nebst Bemer- 

 kuugen zur Epithelregeneration," Archiv f Jr niikrosk. .-Vuat. 25 Bd., 188o ; 

 '• Einiges iiber den Mitteldarni der In>ekten, sowie iiber Epithelregene- 

 ration," Archiv fur rnikrosk. Anat. 26 Bd., l">8(i. 



X I have noticed in sectious of Antaeus that the nuclei of the epithelial 

 cells of the niid-giU, in certain regions lying in the deptlis of the folds, 

 have the appearance of young nuclei, wliich probably divid'? by mitosis. 



§ The way iu which the sncretion collects in tlie cells of the mid-gut 

 of Insects, and how such cells, with lh','ir nuclei, are cast off into the 

 lumen of the intestine when the secretion is poure^l forth, has been 

 minutely described by A. run Gehuchten (" Recherches histologiques sur 

 I'appareil digestif de la larve de PUjchoptera coiifaminata,'^ La Uellule, 

 t. VI. 1890J. Mingazzini, tcjo, alludes to the casting off of the epithelial 

 cells (" Ricerche snl canale digerente dei i^arnellicorni fitofagi," Mitt. a. 

 d. zool. Station zu Neapel, ix. Bd., 1889, pp. .jo and 279). 



25" 



