240 



AYERS ON THE DEVELOPMENT 



K M 



Vc 



The folds which project inward from the antennal region at length extend far forward and 

 unite in the median line, thus bounding a V-shaped area the apex of which is near the 

 front end of the embryo. Near the angle of this depressed area there appears a shallow, 

 cup-like cavity, — the beginning of the stomodaeum. 



The mandibles are now seen as definite oval prominences, while the five 

 pairs of appendages posterior to them appear in optical cross-section to be 

 composed of two concentric circles of cells. This appearance is produced 

 by the two apposed cell layers, ectoderm and mesoderm. The appendages 

 grow out perpendicularly to the germinal band, but later are deflected 

 towards the abdominal region until they finally come to lie in a plane parallel 

 with the longitudinal axis of the body. At an early stage they are some- 

 times to be seen slightly bent toward the head (pi. 18, fig. 9). As the 

 appendages grow out, the egg begins to increase in volume and the cracks 

 in the chorion grow less distinct. Soon after the mesoderm has extended 

 IfpC (., .■ •;.- into the hollow appendages, there appear successively a varying number of 

 ^0 J abdominal protuberances exactly similar to the maxillary and thoracic 



/ 3llo • appendages in their earliest stage of growth. Of these only two pairs ever 



reach any considerable degree of development, they are the first and the 

 last abdominal. The former grows to the length of the mature mandibles 

 and then atrophies. It varies in shape from a finger-like process (pi. 18, 

 fig. 17) to a lobed outgrowth,and in the later stages is covered by the last 

 thoracic appendage. The last abdominal appendage is the primitive anal 

 stylet and acquires a very complex structure which will be described in con- 

 nection with the hatched insect. 1 The intermediate prominences never pass 

 beyond the simple knob-like stage. The mesoderm extends into all of them as an inner 

 layer apposed to the ectoderm, such as has been described for the maxillary and thoracic 

 appendages. 



About the time of the beginning of the invagination for the stomodaeum, there may be 

 detected both in fresh and in hardened embryos a more or less sharply defined linear 

 depression (the Primitivfurche of Hatschek), extending the entire length of the embryo. 

 It is most distinct in the maxillary and thoracic regions and grows gradually fainter as it 

 advances toward the tip of the abdomen just before reaching which it bifurcates, and the 

 resulting lines, after extending for a short distance along the region of the caudal enlarge- 

 ment, approach each other and by their coalescence surround a pear-shaped, depressed 

 area. This line first appears in the thoracic region and grows both ways. It ends in front 

 just behind the mouth opening, while its posterior cup-like termination indicates the posi- 

 tion of the future proctodaeum. The line itself is the superficial indication of a longitudi- 

 nal invagination to form the nervous system. The evidences of this invagination, which 

 appears thus early in the development of the embryo, persist for a considerable time. 



The upper lip appears simultaneously with the invagination for the stomodaeum and 

 arises as a flap or fold in the median line between the bases of the antennae, from the 



Fig. 24. 

 A lateral view 

 of a young e.Tl- 

 bryo still within the 

 fresh egg. Certain 

 bodies apparently of 

 a protoplasmic na- 

 ture are seen at the 

 head end of the egg; 

 no nuclear structure 

 was distingushable 

 within them. X50. 



1 Compare this embryonic stage with the active larva of 

 Lisyra fuscata, a Neuropteron which has all the segments of 



the body provided with articulated appendages. 



