NO. 8 INSECT ABDOMEN SNODGRASS 9 



works on insect embryology contain explicit descriptions of the sup- 

 posed origin of the germ cells from the mesoderm. 



The most detailed account of the history of the insect germ cells 

 after their appearance in the mesoderm is that given by Heymons 

 (1892) in his description of the development of the female reproduc- 

 tive organs of Blattella (Phyllodromia) germanica. The germ cells 

 of Blattella, according to Heymons, appear as a few large cells scat- 

 tered in the posterior parts of the mesoderm at the time when the 

 germ band consists only of ectoderm and a simple underlying meso- 

 dermal layer. At the time the appendages are formed the germ cells 

 are arranged in two lateral series behind the first segment of the 

 abdomen, and most of them occur above the intersegmental grooves. 

 With the development of the coelomic sacs, therefore, the germ cells 

 are situated in the dorsal parts of the dissepiments between the coe- 

 lomic cavities. They extend now from the first to the seventh ab- 

 dominal segments. 



Within the coelomic septa the germ cells multiply, and soon, Hey- 

 mons says, many of them are crowded out posteriorly into the coelomic 

 cavities. The extruded cells remain in contact with the coelomic walls 

 and migrate posteriorly upon them to the splanchnic walls of the sacs. 

 Here they penetrate between the epithelial cells, and thus become 

 again imbedded in the mesoderm. Wheeler (1893) ^^^ observed that 

 at this stage in Conocephalus (Xiphidium) some of the germ cells 

 may become detached and fall into the coelomic cavities. 



Origin of the gonads. — The surface of the splanchnic epithelium, 

 where the latter is penetrated by the migrating germ cells, becomes 

 broken, and its cells grow out irregularly about the germ cells (fig. 

 3 A) , which eventually they enclose in a loose mesodermal covering. 

 The germ cells of Blattella, Heymons says, now form a continuous 

 series on each side of the body, extending from the second into the sixth 

 abdominal segment, the cells that remained in the dissepiments filling 

 the gaps between those that were extruded and which later took a 

 position in the segmental areas of the coelomic walls. The series of 

 germ cells and their mesodermal coverings, projecting slightly into 

 the body cavity, constitute the genital, ridges of the embryo, which 

 are the primitive rudiments of the gonads. 



Wheeler (1893), in his study of the development of the gonads in 

 Conocephalus, was not able to distinguish the germ cells from the 

 mesodermal cells until the germ cells are clustered in segmental groups 

 in the splanchnic walls of the coelomic sacs, where they are partially 

 covered by a mesodermal epithelium. In Conocephalus at this stage 

 the paired gonad rudiments occur in the first to the sixth abdominal 



