344 



INSECTA. 



Phyllodromia germanica, on which, as the most detailed, we shall 

 found our description.* 



In Phyllodromia, distinct genital cells can be distinguished at an 

 early stage of embryonic development by their different histological 

 character. They are larger than the other cells, and show a slightly 

 stainable nucleus with a distinct nucleolus. These genital cells, 

 which have developed by the transformation of the embryonic 

 mesoderm -cells, originally lie in the splanchnic layer or on the 

 surface of this layer, which is turned towards the food -yolk at 



f S A 



- j& 



Fig. 170. — Transverse section through the abdomen in a somewhat older germ-band of 

 Phyllodromia germanica (after Heymons). by, rudiment of the ventral chain of ganglia ; 

 c, remains of the coelomic cavity ; cz, rudiment of the efferent genital duct ; ec, ectoderm ; 

 ef, terminal filament ; en, entoderm ; fk, fat-body ; gz, genital cells ; h, rudiment of the 

 heart; p, rudiment of the pericardial cavity; ps, rudiment of the pericardial septum; 

 so, somatic mesoderm; sji, splanchnic mesoderm. 



the boundaries of the segments. After the coelomic sacs are com- 

 pletely formed (Fig. 168, gz), they are found in the dissepiments 

 which divide the consecutive sacs from one another. New genital 

 cells continually develop here from mesoderm -cells. These cells 

 develop in the second to the seventh abdominal segment, f 



* The following description and figures were taken in advance from a treatise 

 since published in the Zeitschr. f. wiss. ZooL, Bd. liii. , kindly placed at our 

 disposal by the author. 



t [According to recent researches on the origin of the germ-cells in Crustacea 

 and other Invertebrate, we should have to look for the first origin of these cells 

 at an early cleavage-stage, at any rate, they should be visible as soon as the 

 mesoderm-bands are formed. Further, we should not expect them to arise or to 

 increase from the ordinary mesoderm-cells, but from germ-teloblasts. — Ed.] 



