TRICHOPTERA AND LEPIDOPTERA 353 



42 hours are not distinguishable from the rudiments on abdominal seg- 

 ments one, two, seven, eight, and nine, soon develop rapidly, while the 

 rudiments on the segments mentioned become reduced. 



The mid-gut epithelial strands fuse on the longitudinal mid-ventral 

 line at 84 hours, but dorsally the edges are still far apart (Fig. ^I2,mge). 

 The middle portion of the dorsal body wall is likewise still open (umbili- 

 cal passage) , although at the ends the amniotic folds have already closed 

 over the dorsum (Fig. 310). Five hours later the edges of the amniotic 

 folds, which at 82 hours (Fig. 310, amf) were still far apart, have fused, 

 completing the dorsal wall (Fig. 311). 



The cardioblasts arise on the lateral margins of the somatic mesoderm. 

 At about 92 hours they are to be seen in cross section as a small group of 

 cells arranged crescent-like with the convex side directed laterad and 

 blood cells lying between the horns. Posteriorly, in the region of the 

 hind intestine, the horns of the crescentic group of cardioblasts have fused 

 to complete the heart. Two or three hours later, the heart and aorta 

 are also completed anteriorly; the edges of the splanchnic mesodermal 

 layer likewise meet dorsally, completing the mid-gut. Blood cells are 

 abundant in the heart. Communication between the mid-gut and the 

 stomodaeum and proctodaeum is established at this time by the dis- 

 appearance of the stomodaeal and proctodaeal membranes. The closure 

 of the dorsal wall of the mid-gut at 94 to 95 hours results in the inclusion 

 of yolk. From about the thirtieth hour, when the embryo begins to 

 sink into the yolk, until the rupture of the amnion a few hours before 

 emergence of the larva, the embryo is immersed, the peripheral yolk being 

 about the thickness of the diameter of one yolk spherule during the 

 greater part of this time. This peripheral yolk which is present at the 

 time of the rupture of the amnion is soon consumed by the young larva a 

 few hours before the rupture of the chorion. At first, after the com- 

 pletion of the dorsal wall of the mid-gut, there are still yolk cells to be 

 found, both in the peripheral yolk and in that contained in the alimentary 

 canal, but later both yolk masses are liquefied, and the cells are no 

 longer present. No yolk is to be found in the esophagus or in the hind 

 intestine. 



When the dorsal body wall is closed, the embryo rotates about its 

 longitudinal axis until the ventral surface is directed toward the center 

 of the egg. The rotation is about half completed at 89 hours. In the 

 82-hoar stage the amniotic fold has already extended beyond the tips of 

 the stomodaeum and proctodaeum. In Fig. 310 the proctodaeum has 

 been cut somewhat obliquely, owing to the slightly spiral position of the 

 embryo in the egg. A cross section through the proctodaeum of an 

 embryo of the same age is shown in Fig. 311, indicating how the inner 

 fold of the amnion forms a provisional dorsal wall — a condition also to 



