126 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



The later history of the genital rudiment consists of a differentiation 

 of the inner cellular elements; the formation of the definitive sperm 

 tubes, or the so-called testicular "follicles"; the establishment of the 

 sperm tube ducts (vasa efferentia) ; and the formation of the gonadial part 

 of the vas deferens. The following is a summary of testis development 

 in Melanoplus differentialis given by Nelsen (1931). 



At the beginning of the postrevolution period in the development 

 of the embryo, the genital rudiments have somewhat shortened, reaching 

 now only to the end of the seventh abdominal segment. By the middle 

 of this period, the indifferent mesodermal cells of each gonad begin to 

 differentiate into connective-tissue elements which form the intratesticu- 

 lar partitions among the germ cells, thus segregating the latter into groups 

 of one or more cells each. The partition-forming cells are probably 

 generated from cells at the junction of the gonad proper with the ventral 

 cell strand. The actively proliferating area here located is termed by 

 Nelsen the "germinal center." Each cell group, or cell nest, thus second- 

 arily isolated, marks the nucleus of a definitive sperm tube, or "follicle." 



By the time of hatching, the gonads have the form of two cords lying 

 immediately below the heart, extending from the rear half of the third 

 abdominal segment into the anterior half of the sixth. The germ cells 

 have multiplied until there are about four or five cells in each group. 

 Now, the "indifferent mesoderm cell," Nelsen states, leaves the periphery 

 of the gonad and pushes into the center of each germ-cell group where it 

 sends out cytoplasmic processes that fill the spaces between the germ cells. 

 This interpolated cell becomes the apical cell of the group; the entire nest 

 of cells is surrounded by a capsule of connective-tissue cells. The whole 

 formation, which will be retained in the apex of each sperm tube, Nelsen 

 calls the "apical complex." The growth of the apical complexes causes 

 a series of lobes to appear on the dorsal surface of the gonad, which are 

 the beginnings of the definitive sperm tubes. Undifferentiated cells now 

 grow upward from the germinal center against the lower end of each 

 apical complex, while at the same time the outer epithelial sheath grows 

 inward between the dorsal lobes of the gonad until it invades the germinal 

 center. In this manner the young sperm tube is formed, consisting of a 

 germarium, which is the apical complex, and of a duct (vas efferens) 

 derived from the germinal center, the whole structure invested in a fold 

 of the outer epithelial sheath. The further activity of the gonad, which 

 takes place during postembryonic life, is the formation of the sperm cysts 

 and the further differentiation of internal cellular elements. 



The primitive mesodermal exit ducts of the gonads become the lateral 

 oviducts in the female and the vasa deferentia in the male, except in so 

 far as they may be partially replaced by branches of the ectodermal 

 median ducts. Since the mesodermal ducts are derived from the coe- 



