INVOLUTION OF THE EiMBRYONIC ENVELOPES. 



305 



tion ; the degenerating serosa-cells lose their epithelial connection, 

 and in this disintegrated condition are absorbed into the intestinal 

 canal with the rest of the food-yolk. Simultaneously with this 

 disintegration, which leads to the complete degeneration of the 

 dorsal organ, the outer aperture of invagination completely closes. 

 In this way the serous part of the wall of the yolk-sac becomes 

 disintegrated. There now only remains the amniotic portion of this 

 wall, which, standing 



in direct communication 

 with the ectoderm of 

 the embryonic rudiment, 

 represents a provisional 

 dorsal integument. It 

 still appears doubtful to 

 what extent this pro- 

 visional integument 

 passes over into the 

 permanent wall, i e., to 

 what extent the amnion 

 is transformed into the 

 definitive hypodermis (a 

 view which Graber 

 <(Xo. 27) and others 

 have been disposed to 

 -adopt). It would ap- 

 pear very strange if 



A 



ar 



Fio. 149. — Three embryos of Hydrophilus from the dorsal 

 side (after Kowalevskv, from Balfour's Text-book). 

 A, the serosa has retracted to the dorsal side and has 

 thickened to form the dorsal plate {do). B, the dorsal 

 plate (do) is partly invaginated and covered by the 

 amnion (Fig. 150 D). C, the dorsal tube is completely 

 developed and opens externally only through an anterior 

 pore (c/. Fig. 150 E). at, antenna; do, dorsal organ in 

 various stages of development. 



the permanent dorsal 



integument were to be utilised in earlier embryonic stages as a 

 provisional ventral embryonic envelope (amnion), and as, on the 

 other hand, as we shall show (p. 307), the degeneration of the 

 amnion was directly observed by "Wheeler in Doryphora, we must 

 leave the question open whether, as a rule, in the Insecta, the germ- 

 hand alone forms the whole of the embryonic rudiment, and also 

 brings about, by its dorsal extension and subsequent union, the 

 completion of the permanent dorsal integument, while the amnion 

 serves as a provisional integument, which later undergoes gradual 

 absorption. 



The above-described process of the completion of the dorsal body-wall by 

 the development of a dorsal organ and provisional completion by means of the 

 amnion, probably applies to the Libcllulidac. It is also found in all Rhyncota 

 <Gkaber, No. 27, in Pyrrhocoris ; Metschnikoff, No. 55, and Brandt in 



X 



