122 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



Japyx, Machilis, Lepisma, and related genera and also of the Diplopoda 

 offer an exception. They are derived from the mesoderm and in struc- 

 ture resemble nephridia. The first anlage of these mesodermic glands 

 appears early in the second maxillary segment at the level of the ganghon 

 as a vesicle with distinct epithehal walls. The efferent duct alone is of 

 ectodermal origin. The tubular mesodermic glands are wanting in both 

 the Chilopoda and the Pterygota. 



The manner in which the anlage of the tubular head glands (Fig. 

 98, hg) develop and especially their derivation from the mesoderm 

 substantiates the view of Philiptschenko (1912) that they are retained 

 labial nephridia. Thej^ are therefore fully comparable to similarly 

 retained nephridia of other arthropods and of Peripatus. They develop 

 exclusively from mesoderm except for a portion of the duct which is 

 ectodermal. 



THE REPRODUCTIVE ORGANS 



It is noteworthy that the anlagen of the internal parts of the reproduc- 

 tive organs in many instances are spread over a number of abdominal 

 segments in insects and to a still greater extent in the chilopods. In 

 the dorsal part of the splanchnic layer of the segments a genital ridge is 

 developed laterally (Figs. 17, 18, gr-) for the reception of the germ cells 

 that have been differentiated at the caudal end of the egg and migrate 

 forward as has previously been described. At first this ridge is formed in 

 several segments — in some cases, as in Pyrrhocoris, in segments one to 

 eight. So long as the primitive somites are still intact, the germ cells 

 adhere to the genital ridges. While the germ cells continue to develop, 

 the genital ridges of the somites of each side unite into a single longitudina 

 band of cells, one on each side. The development of the genital ridge 

 into a gonad occurs when the germ cells that are distributed through a 

 number of segments migrate forward and become enveloped by the cells 

 of the mesodermal genital ridge of that region. The manner in which 

 this consolidation of the germ cells and the development of the gonad 

 around them is accomplished differs with the group to which the insect 

 belongs. In any case the germ cells are fully covered by the mesodermal 

 cells, the former giving rise to spermatozoa, or eggs, and nurse cells, the 

 latter to the epithelial envelope. The account of Lautenslager (1932) 

 of the development of the female gonads of Solenohia triquetrella (Psy- 

 chidae) is in essential agreement with the observations of earlier writers 

 on Orthoptera. The paired segmental groups of germ cells, lying in the 

 fourth, fifth, and sixth abdominal segments, condense and unite on each 

 side into a single group having the position of the definitive gonad in the 

 fifth segment. Here each composite group of germ cells becomes enclosed 

 in a layer of small mesodermal cells. The latter form a thin sheath 



