OLIGOENTOMATA AND APTILOTA 



189 



development of the larvae further than this point, but in some captured 

 specimens only a little larger than the reared ones the mid-gut epithelium 

 had already assumed its definitive form, and he therefore assumed that 

 further proliferation of the clusters of yolk cells gave rise to the mid-gut 

 lining. The definitive lining (digestive epithelium) consists of rather 

 large cylindrical cells between which at intervals are clusters (nidi) of 

 smaller, closely crowded replacement, or regenerative, cells. As no 

 proliferations of cells at the tips of either stomodaeal or proctodaeal 



h 



crypt 



mus 



crypt 



ggl.7 



Fig. 112. — Lepisma. Cross section of seventh abdominal segment. (/) Fat. iggl) 

 Seventh abdominal ganglion, (h) Heart, (mus) Muscle, {at) Stigmata, (y.nu) Yolk 

 nucleus. 



invaginations take place, Heymons (1897) concludes that in Lepisma the 

 mid-gut epithelium is of entodermal origin. 



OBSERVATIONS ON THE DEVELOPMENT OF APTERYGOTA IN GENERAL 



Cleavage in Campodea and Lepisma is superficial; in Isotoma, Anurida, 

 Achorutes, and Macrotoma, as in many arachnids and crustaceans, it is at 

 first total but later becomes superficial. 



The entoderm in Campodea, according to Uzel, arises from a thick ring 

 of cells at the ventral pole of the egg, the cells later migrating to the inner 

 wall of the blastoderm and thence into the yolk. The yolk cells are the 

 source of the mid-gut epithelium in Lepisma according to Heymons (1897). 

 In Macrotoma and Achorutes cleavage cells migrating back from the 



