NO. 14 INSECT ABDOMEN SNODGRASS 7 



and Protura ; the process of segment formation has been followed in 

 detail by Pflugfelder (1932) in a diplopod. If the generation of 

 arthropod somites is invariably teloblastic, it would seem to follow 

 that the numerical order of a segment in the body series of any 

 arthropod would determine the homology of this segment with a seg- 

 ment of the same number in any other arthropod. This principle, 

 however, does not apply to the anterior segments, since in the de- 

 velopment of both the annelids and the arthropods there is a pri- 

 mary body region that becomes itself differentiated into a small num- 

 ber of somites. With the Polychaeta the first few body somites are 

 formed directly in the hyposphere of the trochophore ; the nauplius 

 larva of Crustacea has a short body region containing two primary 

 somites (second antennal and mandibular), the succession of secon- 

 dary teloblastic somites beginning with the segment of the first 

 maxillae. (See Iwanoff, 1928; Sollaud, 1923.) 



ORIGIN OF THE GONADS AND THE GONODUCTS 



The groups of primary germ cells that occur in the mesodermal 

 tissues are known to students of the Annelida as the " gonads ", and 

 they are appropriately thus termed since the word means a " sprout " 

 or " germinating bud ". These germ cell groups of the annelids, how- 

 ever, correspond with the so-called " germaria " of the Arthropoda, 

 and in arthropod anatomy the term '* gonad " refers to the meso- 

 dermal sac that contains the germinal cells in its epithelial walls. 

 This nomenclatural confusion is unfortunate, and it is difficult to 

 remedy. For an arthropodist it is easier to use the term gennarium 

 in a general sense to include the annelid " gonads ", and to retain the 

 name gonad for the mesodermal sacs (ovaries or testes) that contain 

 the germaria — a usage generally followed in zoology. 



In the Annelida the primary germ cells are early localized in the 

 coelomic epithelium, where, as they multiply, they form small cell 

 masses, which may occur most anywhere in the coelomic walls or may 

 be limited to definite areas of particular segments. The germ cells 

 given ofif from the germarial centers undergo their development in 

 the coelomic cavities or in coelomic pouches. In some forms the sex 

 elements escape through temporary openings in the body wall, in 

 others the posterior part of the body containing them is constricted 

 off, but more generally they make their exit through special genital 

 outlets, or through modified nephridia. 



In the Onychophora the germarial centers of the embryo are located 

 in the splanchnic walls of the mesoderm above the alimentary canal 



