ECTODERMAL DERIVATIVES 107 



ium, Eutermes, and Calandra (oryzae) and from the epidermis between 

 the lobes by the conversion of some of its cells into neurogene tissue in 

 Forficula, Pieris, Corynodes, and Apis. The tritocerebral commissures are 

 said to come from the lobes in Locusta and Pieris and from the epidermis 

 between the lobes similar to the ventral cord in Eutermes, Forficula, 

 Calandra (callosa), Chalicodoma, and Apis. In general, no commissure 

 is formed between the lobes of the deutocerebrum, or at least it is not 

 distinct from that of the protocerebrum. Mantis, according to Yiallanes 

 (1891), offers an exception. The neurilemma of the brain in Mantis 

 develops from some surface gangUon cells. Certain points in which the 

 development of the brain of the honeybee differs from the Orthoptera 

 and Dermaptera are as follows: The cells of the second generation from 

 the neuroblasts, instead of the first, form the definitive nerve cells; the 

 three subdivisions of the protocerebrum are not at first plainly marked 

 off from one another and are never separated by hypodermal ingrowths ; 

 finally, the optic lobes are formed by a deep invagination of the neurogenic 

 ectoderm, as in the formation of the simple eyes in the beetles Acilius 

 and Hydrophilus and in Vespa. 



In its fundamental features the development of the nervous system 

 in the collembolan genus Isotoma, including the formation of brain lobes, 

 the development of the ganghon cells from neuroblasts, etc., is identical 

 with, that of pterygote insects. Although Philiptschenko (1912) failed 

 to find it in Isotoma, Claypole (1898) notes the presence of an anlage of a 

 sympathetic system in Anurida. Nor, in general, does the development 

 of the nervous system in the chilopod Scolopendra differ greatly from 

 that of the insects. 



The Sense Organs. — Integumental sense organs distributed over 

 various parts of the surface of the body, usually simple in structure con- 

 sisting of but few cells, are modified epidermal cells functioning as tactile, 

 gustatory, or olfactory structures. In their simpler form they consist of 

 either single cells or a group of a few cells which distally develop a sensory 

 style and inwardly are provided with a nerve. In late embryonic life 

 certain ectodermal cells become conspicuous by reason of their greater 

 size or other modifications, e.g., the tactile bristles of caterpillars which 

 develop in the embryo from trichogen cells. These cells (Fig. 311, 

 trich) in some cases have become so enlarged as to extend far below the 

 level of the adjacent epidermis. The olfactory and gustatory cells on 

 the antennae and palpi more often develop in groups which may be found 

 within pits. The auditory and static organs, located on the body or 

 legs which are still more complex in structure, are likewise of ectodermal 

 origin. But little is known of their embryonic development. 



Most complicated are the visual organs. The compound eyes and 

 the ocelH of paurometabolous and hemimetabolous insects and the simple 

 eyes (adaptive ocelli) of the holometabolous insects are at least in part 



