DEVELOPMENT OF PHRYGANIDS. 593 



median line of the ventral plate. This has been observed in 

 all cases where the development has been carefully studied. 

 Kowalevsky was the first to call attention to it, and his obser- 

 vations have been confirmed by Graber, 1877, and the Hertwigs, 

 1881. In the Phryganids, all the mesoderm arises from 

 this invagination. Tichomeroff, 1879, claims that the 

 mesoderm is not formed by an invagination, but by budding 

 from the cells of the blastoderm and primitive ventral plate. 

 Since the gastrula stage is of such short duration, it was 

 probably not included in the eggs which he sectioned, and the 

 cells observed to bud from the ventral plate and blastoderm 

 are in reality endoderm, instead of mesoderm cells. 



Graber had already claimed, in 1871, that some of the endo- 

 derm cells were formed from the blastoderm. Balfour, how- 

 ever, discredits his statements and believes that the phenomena 

 observed by Graber were in reality due to the passage of yolk- 

 cells into the blastoderm. It is very probable, however, that 

 Graber is right in his conclusions, and that he observed the 

 same process that we have described for Phryganids. In this 

 case, however, we have more conclusive evidence, for all the 

 GERM CELLS having migrated to the surface, it is much easier 

 to observe a passage of cells from the blastoderm into the yolk. 

 In the later stages, where yolk-cells apparently bud from the 

 mesodermic segments, it is not posssible to state with the same 

 confidence the direction in which the cells are migrating. The 

 facts, however, seem to indicate, as we have already stated, 

 that some of the yolk-cells are formed at this late period by 

 budding from the mesodermic segments. This does not appear 

 so improbable when we reflect that the segmentation of the 

 yolk, a process which ordinarily precedes the formation of the 

 endoderm, occurs also at this period. 



The dorsal organ, or something equivalent to it, is found in 

 all insects whose development has been carefully investigated. 

 Uljanin and Bobretzky have described the dorsal organ of 

 Amphiphods as a saddle-shaped area of cells attached to the 

 vitelline membrane, and extending over the dorsal region of 

 the body; the whole layer becomes disconnected from the 



