MYRIAPODA 



401 



The organ is in all probability homologous with the dorsal organ of the 

 apterygotes. Its function is problematical. 



The Reproductive Organs. — The dorsal extensions of the coelomic sacs 

 remain intact as a pair of contiguous, flattened, longitudinally chambered 

 genital tubes below the heart (Fig. 357, g. coel). More room is provided 

 for the development of the parts lying above by the reduction in the 

 diameter of the mid-gut because of the diminution of the yolk within, the 

 genital tubes becoming circular in cross section (Fig. 358, g. coel). As 

 development proceeds, single cells with larger nuclei and more abundant 

 cytoplasm may be observed in the genital anlagen, or gonads. These 

 cells are germ cells as well as cells that will be future follicular epithelium. 



n g 



coel 



Fig. 358. — Scolopendra cingulata. Cross section of late embryo, {hi) Blood cell, {ent) 

 Entoderm, (ep) Epidermis, ig.coel) Genital coelome. (n) Dorsal nerve, (pc) Peri- 

 cardial cell (pericardial fat cells not yet distinguishable from these), ipm) Pericardial 

 membrane, {splm) Splanchnic mesoderm, (y) Yolk. {Adapted from Heymons.) 



It is a characteristic that, with those insects in which germ cells are 

 recognizable at an early stage, they form as a mass at the posterior end 

 of the egg or of the germ band and then wander or are pushed forward 

 into the middle or anterior abdominal region where they undergo further 

 development. In Scolopendra, at the posterior end of the germ disk, a 

 mass of cells hkewise appears, which, though not yet recognizably dif- 

 ferentiated, later migrate forward to be eventually enclosed in the 

 epitheUal layer of the coelomic sacs. These cells then find a place 

 especially on the ventral and mesal walls of the genital tubes. The mesal 

 walls of the two contiguous tubes as well as the division walls of the 

 longitudinally placed chambers break down, resulting in the formation 

 of a single elongated median gonad, which is outwardly clothed with a 

 muscular layer and inwardly lined with regularly arranged, genital 

 epithelial cells. 



