No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 6 1 



Many attempts have been made to explain the origin of the 

 amnion in insects. It first appears abruptly and fully devel- 

 oped in the Orthoptera just as the vertebrate amnion appears 

 abruptly in the Reptilia. One school, represented by Nus- 

 baum (-87) and v. Kennel ('85, '88), regards the insect amnion as 

 a structure of high phylogenetic value and would trace it to 

 some organ in the lower Arthropods or in the worms. Ac- 

 cording to another view advocated by Will ('88) and myself 

 (■89), the amnion has had no such remote phylogenetic history, 

 but has arisen more recently in response to certain purely 

 mechanical conditions of development. 



Nusbaum advances the opinion that the cellular envelopes 

 of the insect embryo are homologous with the "dorsal organ" 

 of Crustacea. The saddle-shaped "dorsal organ" of Ligia and 

 Oniscus is regarded as the key to this homology, the two flaps 

 which clasp the sides of the Isopod embryo being equivalent 

 to undeveloped amnioserosal folds. But I have shown in the 

 present paper that the indusium of Xiphidiiim is very probably 

 the homologue of the crustacean "dorsal organ," and as there 

 is besides a well developed amnion and serosa in XipJiidiiim, 

 Nusbaum's hypothesis must fall to the ground. His assertion 

 was certainly premature that the " deux series des organes aussi 

 characteristiques que le sont I'organe dorsal et les enveloppes 

 embryonnaires, s'excluent reciproquement dans les deux 

 groupes des Arthropodes, c'est-a-dire chez les Tracheates et 

 les Crustaces." 



So far as the insect envelopes are concerned v. Kennel's 

 views do not differ essentially from Nusbaum's. He likewise 

 homologizes the crustacean "dorsal organ" and the Poduran 

 "micropyle" with the Hexapod amnion and serosa. But he 

 goes further and includes under the same homology the 

 amnion of Pcripatiis, Scorpions and Chelifer and the chitinous 

 envelopes of Myriopods. He supposes all these structures to 

 represent remnants of the annelid trochophore. I feel con- 

 fident that he has jumbled together at least three categories of 

 organs which cannot be regarded as homologous inter se, viz. : 

 (i) the series of structures typically represented by the Crus- 

 tacean "dorsal organ"; (2) the cellular envelopes of insects; 



