rn til)' ^iiiuiiul /v I. II (/do III. 37 L 



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

 themselves are found in all sta<^es of direct division. 



The coiiditioiis presented by the epithelium of the mid-i^iit 

 of Insects * and Crustacea re(|uire special discussion. After 

 a ciitical 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 

 [jroceetls from isolated grou[)s 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, 

 J\laja, and Dromia he observed typical anut(jtic nuclear divi- 

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

 cells of the mid-gut have to jjerform 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 jierish in so doing " §. " The 

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

 whether ihey belong to the actual intestinal tube or to its 

 evaginations, 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." !So far Frenzel's statements agree very well with 



* Amitotic nuclear division occurs uot ouly in the mid-gut, but also in 

 the hind-gut of Insects. Faussek ( " Beitrjige zur Histologie des Darm- 

 kanals der In.S'^kten,'' Zeitschr. f. Aviss. Zoologie, lid. 4-j. 1887) observed 

 it in the hind-gut of a gr.isshopper {Eremohia nmrknta, Pall.) and in the 

 rectal glands oi ^^chna-\aY\?p. So far as we know, this division of the 

 nucleus is not followed bv a division of the cell. 



+ J. Frenzel, '• Ueber den Darnikanal der ( 'rustaceen nebst Bemer- 

 kuiigen zur Epithelregeneratioii," Archiv fir mikrosk. .-Viiat. 2ii Bd., 188-) ; 

 '• Einiges iiber den Mitteidariu der In-ekten, sowie iiber Kpithelregene- 

 ration," Archiv fur aiikrosk. Anat. 2(i Bd., L^8(5. 



X I have noticed in sections of A-ifacu-i that the nuclei of the epithelial 

 cells of the mid-gut, in certain regions lying in tlie depths uf the fulds, 

 have the appearance of young nuclei, which probably divid'A by mitosis. 



§ The way in which the secretion collect;* in tlie cells of the mid-giit 

 of Insects, and how such cells, with their nuclei, are cast off into the 

 lumen of the intestine when the secretion is poured forth, has been 

 minutely described by A. van Gehuchten (" Recherches histologiques siu* 

 I'appareil digestif de la larve de Ptychoptera confaminata" La Cellule, 

 t. vi. 1890). Mingazzini, too, alludes to the ca-ting oflf of the epithelial 

 cells (" Riceiche sal caiiale digerente dei l^arnellicorni titofagi,"' Mitt. a. 

 d. Ziiol. Station zu Neaptl. ix. Bd., 1889, pp. .")") and 279). 



2."r 



