OP OECANTHUS AND TELE AS. 241 



region where the antennal folds united. It projects backward and downward so as to 

 partly cover the mouth opening. The caudal enlargement is now greatly changed in its 

 relation to the embryo. It has up to this time been lying in the same plane with the 

 body of the embryo, but by some means at present unknown it is reflected toward the 

 head, so that its dorsal side is uppermost and the ventral surface of the last 3-5 segments 

 of the abdomen lie in contact with the ventral surface of the preceding segments. It 

 forms in this stage a £5 -shaped fold. 



The amnion is now seen springing, not from the tip of the abdomen, but from the region 

 of the last pair of abdominal appendages, the anal stylets ; in other words, the posterior end 

 of the abdomen bas grown backward beyond the limits of the amniotic membrane and lies 

 free in the yolk. Since the force which causes the folding seems to act through the amni- 

 otic layer, it is apparent that this free abdominal tip will not be changed in direction but 

 will be merely drawn forward and displaced to a plane ventral to the body of the embryo, 

 with which it remains parallel. The pear-shaped depression in which the nervous invagina- 

 tion of the median line terminates behind has, by gradually deepening, extended back to 

 the tip of the abdomen, and it is the blind end of this pocket which forms the tip of the last 

 d of the g. (See pi. 18, figs. 19, 20, 21, 22, 29 ; pi. 22, figs. 18-21 and 26. 



Tbe intimate connection of this pocket, or proctodaeum, with the invagination that 

 forms the nervous system is w r orthy of notice. Compare Ganin (19, pi. xxxi, figs. 6, 

 9, 10-12, Platygaster). 



After sprouting out from the body the appendages grow rapidly and soon show con- 

 strictions near their bases. In the case of the mandibles and maxillae the free ends also 

 become lobed. (PI. 18, figs. 20-22.) The three oral appendages are trilobed ; the 

 lobation is most prominent in the second maxillary and least in the mandibular appendage. 

 The primitive appendage is first divided into two lobes and the inner of these becomes 

 secondarily divided into two. The antennae and thoracic appendages grow with equal 

 rapidity until the time of revolution, when the antennae suddenly commence an extremely 

 rapid growth and soon acquire a length equal to twice that of the body. During this 

 rapid growth there is a gradual decrease in the diameter of the appendage, and constric- 

 tions appear in its walls at irregular intervals. The upper lip is now a broadly ovate flap 

 and entirely conceals the mouth opening. The first pair of abdominal appendages has 

 reached its maximum development (pi. 18, figs. 22 and 23), whereas the last abdominal 

 pair is scarcely more advanced than the nine intermediate rudimentary appendages, which 

 are now prominent elevations of the body wall and show the mesodermic or inner layer 

 and a central lumen, when seen in optical section. 



The stomodaeum has so far advanced as to project some distance beyond the inner wall 

 of the body, its blind end lying free in the yolk. It is a circular tube with a distinct 

 lumen extending from the mouth opening to the blind end. Its wall is composed of cylin- 

 drical epithelial cells. The proctodaeum has grown but little in length, and, like the 

 stomodaeum, its free blind end extends into the yolk, while its lumen communicates by 

 means of the anal opening with the amniotic cavity. Its anal end is bordered on either 

 side by the enlarging anal stylets. 



In cross sections of embryos of the stage represented in pi. 18, fig. 17, the median line 

 seen on the ventral surface of the embryo is shown to be the outer ends of cells whose 



MEMOIRS HOST. SOC. NAT. HIST. VOL. III. 31 



