110 CAROLINE BURLING THOMPSON 



Sections show also that the brains are differentiated into the 

 central fibrous core and the enveloping nerve cell layer, all the 

 cells of which are alike and undifferentiated at this age, and in 

 which mitotic figures are frequent. 



All the parts of the brain which are present in the adult are 

 found in the brains of the newly hatched nymphs, and with little 

 difference from the adult except in size, with the one exception 

 of the optic lobes, which, in both types, have but one fiber mass, 

 instead of the three of the adult. 



The head of the newly hatched nymph, as seen in frontal sec- 

 tion, (figs. 33, 34, 41) is covered by a very thin cuticula, scarcely 

 thicker than the layer of hypodermal cells which has secreted it. 

 Within the head, and attached to the hypodermis on each side 

 of the median line, are two broad bands of very slender muscle 

 fibers which are the beginnings of the great mandibular muscles, 

 the m. adductor magnus mandibulae of Holmgren. 



The frontal gland — a gland found near the postero-dorsal 

 surface of the brain, and which in a former paper (Thompson, 

 '16), I have described as a degenerate median ocellus — is very 

 slightly differentiated at the time of hatching and is barely 

 distinguishable. It consists, in individuals of the reproductive 

 type, of a very slight invagination of the hypodermal cells in 

 the median line of the frontal surface of the head. In frontal 

 sections, (fig. 33, f.g.) the invagination lies above the more 

 frontal portion of the brain. Beneath the hypodermal invagina- 

 tion mesenchym cells form a delicate membranous sheath, the 

 future basement membrane of the frontal gland, which extends 

 backward to the posterior limits of the brain; the hypodermal 

 invagination, however, has a much smaller extent, and is present 

 only in three sections. In figures 34 f.g. the empty cup-shaped 

 basement membrane is seen, at some distance posterior to the 

 hypodermal invagination. Attached to the sides of the base- 

 ment membrane are two slender muscles, the m. retractor 

 fontanellae of Holmgren, which run down behind the brain to 

 their distal attachment to the tentorium. In individuals of the 

 worker-soldier type there is as yet no hypodermal invagination 

 of the frontal gland, and the only traces of this organ are the 



