262 INVERTEBRATA CHAP. 



in Blatta also the endoderm has been derived from the cells of those 

 bands to which the blastoporal pit gives rise. 



Heymons (1894) has, however, drawn a completely contrary con- 

 clusion from his study of Orthoptera. According to him the thin 

 sheets of cells attached to the stomodaeum and proctodaeum (end, 

 Fig. 202, A and B) have arisen by the proliferation of the inner ends 

 of those structures, and are therefore ectodermal in character ; he 

 thus draws the conclusion that the " endodermal bands " must, 

 therefore, also have an ectodermal origin, and that for the same 

 reason the epithelium lining the whole of the alimentary canal of 

 the higher insects must be ectodermal in character. Finally, he 

 supposed that the true endoderm, which must have existed in the 

 ancestral insect, was represented in most modern Orthoptera by 

 degenerated yolk cells. 



Heymons and his pupils have sought to show that this conclusion 

 is true for all the higher Insecta, including the Coleoptera, though 

 they admit that in the lower Insecta a true endoderm is present. 



It is the merit of Hirschler to have shown the untenability of 

 this theory, which is at complete variance with what is known from 

 the study of all divisions of the animal kingdom. Wherever experi- 

 ment can be applied (see p. 525), it is always found that ectoderm and 

 endoderm are physiologically differentiated, that they possess different 

 organ-forming substances which make them functionally irreversible. 



Heymons' error is a good example of the kind of trap into which 

 embryologists may fall ; he has chosen a case on which to base this 

 theory where the endodermal rudiment becomes distinguishable only 

 at a very late stage of development, and where its first origin is 

 impossible to determine with accuracy. It is almost certain that 

 he never would have propounded such a theory if he had begun with 

 cases where the differentiation of the layers is a more simultaneous 

 proceeding. The whole of Heymons' theory stands or falls with the 

 assumption that the sheets of cells attached to the inner ends of 

 stomodaeum and proctodaeum are derived from those structures, and 

 of this he gives no proof. 



The embryo of Blatta develops large compound eyes on the 

 cephalic lobes before leaving the egg, but their development has not 

 been minutely studied. Blatta likewise differs widely from Dory- 

 phora inasmuch as the embryo develops rudiments of appendages 

 on all the segments of the abdomen (ab, Fig. 205); of these, the 

 appendages of the first abdominal segment have a glandular structure 

 as in Doryphora, the last pair of the appendages persist as the anal 

 cerci and all the rest disappear. 



The development of the genital organs in Phyllodromia, another 

 genus of Orthoptera closely allied to Blatta, has been worked out in 

 great detail by Heymons (1891). The original genital cells, which 

 we may term primitive germ cells, and which doubtless originate 

 at the hinder end of the embryonic area, as in Donacia, were first 

 recognized as large cells with pale nuclei lying between the yolk 



