OLIGOENTOMATA AND APTILOTA 177 



From the time the chorion ruptures until after completed blasto- 

 kinesis, 5 days have elapsed, or 10 days since the deposition of the eggs. 

 The remaining steps in the development, the completion of body parts 

 and organs until hatching, require on an average 10 more days, or a total 

 of 20 days of incubation. During this final period of 10 days the number 

 of simple eyes on each side increase to eight, and the appendages acquire 

 the definitive number of segments: four for the antennae, five for the legs, 

 and three for the fork. Mandibles and maxillae also increase in size 

 (Fig. 88). The dorsal closure of the body is accomphshed by the growth 

 of the ectoderm to replace the degenerating cells of the amnioserosa. 

 The dorsal organ is taken into the alimentary canal at the time of the 

 closure of the dorsal wall and digested with the remains of the yolk (Fig. 

 89). Shortly before emergence of the insect the dorsal organ as well as 



f 



ac 



Fig. 89. — Isotoma. Oblique, almost frontal section of thorax of late embryo. (,ac) Ali- 

 mentary canal, (do) Dorsal organ. (/) Fatty tissue. 



the remains of the yolk have entirely disappeared from the ahmentary 

 canal. 



The first evidence of digestive epithelium development is seen about 

 10 hours after egg deposition. This cellular layer originates from an 

 anterior, a posterior, and a diffuse median anlage (Figs. 90, 91). Above 

 the stomodaeum is a mass of cells which will give rise to the esophageal 

 muscle (Fig. 90, mus). The apex of the stomodaeum is free from 

 inner-layer cells. Below the stomodaeum (Fig. 90, mge) is another mass 

 which will give rise to the anterior pair of rudiments of the mid-gut 

 epithelium as well as muscles of the lower wall of the esophagus. A mass 

 of inner-layer cells surrounds the tip of the proctodaeum (Fig. 91, mge) 

 which will give rise to the posterior end of the mid-gut epithelium. As 

 has already been described, dorsad of the developing nerve cord there is a 

 sheet of inner-layer cells connecting the right and left coelomic masses 

 (Figs. 81, 82, mds). This is the middle strand. With the development 

 of the nerve cord and the growth of the neural ridge the cells of the middle 

 strand are liberated, some of them migrating to the yolk surface where 



