2G0 



AYERS ON THE DEVELOPMENT 



As has already been shown for Oecanthus, the embryonic membranes (especially serosa) 

 finally assume the condition of the dorsal organ of Hydrophilus, with this notable excep- 

 tion : the organ does not extend the entire length of the mesenteron, nor does it possess a 

 distinct lumen, but as the sequel will prove the difference is only one of degree and not of 

 kind. 



Briefly reviewing the facts in the two cases, we find that in Hydrophilus there is a 

 fusion of the serosa and amnion and a subsequent rupture of the fused membranes in the 

 median ventral line. 1 Fig. 30. 



Fig. 30. 



Fig. 32. 



.gastuio. 



Fig. 35. 



Fig. 36. 



Fig. 37. 



Fig*. 30-34. Diagrammatic illustrations of tho formation of ICowalevski's dorsal organ. 

 Figs. 35-37. Diagrammatic illustrations of the varying relations of the embryo to its yolk sac. 



By the contraction of the serosa the amnion is pulled off from the ventral surface of the 

 body (fig. 31) and goes to form the dorsum of the embryo after the serosa has contracted 

 into a thick rod and has passed into the niesenteron. The serosa first forms a thick plate 

 on the dorsum of the embryo (fig. 32 and Kowalevski, 26, pi. 8, figs. 14, 15, 16). 

 This plate is in union with the body wall at its edges, and by the upgrowth of the walls 

 it becomes longitudinally folded on itself forming a tube open at the head end. (Fig. 

 33.) This end is the last to be covered over by the dorsum. (Fig. 34.) It finally 

 loses its connections with the body walls and undergoes disintegration within the 

 mesenteron. 



In Oecanthus the membranes fuse at or near the head, and by a self eversion through 

 the opening caused by their rupture the embrj'o comes to lie outside of the amnion and 

 serosa. The latter now forms a yolk sac, which by a gradual contraction, as the yolk is 

 absorbed, comes to lie within the body, being last seen just back of the head. (PI. 19, 

 figs. 1—5, ; pi. 22, figs. 1, 11 ; pi. 23, fig. 2 ; pi. 25, fig. 31.) It is always in connection with 

 the mesenteron and its thick walls finally disappear by a process of disintegration. 



Although Brandt (8) makes no mention of the function or fate of the thickened serosal 

 yolk sac which he so frequently figures (loc. cit, pi. 2, figs. 15-22; pi. 3, figs. 32, 34, 

 38), there can be no doubt that it is the same structure that exists in Hydrophilus and 

 Oecanthus. 2 



1 Kowaltvski leaves this point in doubt since lie was the former seems more probable than the latter, 

 uncertain as to the fate of the amnion. He states that it is - Compare also Metschnikolf (32) pi. 23, figs. 15, 19-23; 



either ruptured or absorbed. From its fate in other insects pi. 2G, figs. 9-27 ; pi. 27, figs. 20-28 and pi. 30, figs. 29-34. 



