NERVOUS SYSTEM AND TRACHEAL INVAGINATIONS. 301 



chain of ganglia, two swellings running longitudinally along the germ- 

 band near the median line (primitive swellings, Fig. 147 A, s) and 

 a channel lying between them (primitive groove, neural groove). 

 Segmentation takes place early in the primitive swellings, broader 

 paits (rudiments of the ventral ganglia) alternating with constricted 

 parts (longitudinal commissures) in regular segmental order (Fig. 146 

 .4, g). Anteriorly, the primitive swellings diverge from one another, 

 as the circum-oesophageal commissures, and pass directly into the 

 cephalic lobes. Here each passes into the brain-rudiment, a some- 

 what large ectodermal thickening, the shape of which will be described 

 more in detail below (p. 326). The rudiment of the brain and that 

 of the ventral chain of ganglia are thus, in the Insecta, connected 

 from their first appearance. 



The tracheae arise as ectodermal invaginations recurring in each 

 segment (Fig. 146 and 147, st). The apertures of the invaginations 

 afterwards become the stigmata. The tracheal invaginations occur 

 regularly on the first to eighth abdominal segment. In the thorax, 

 in which the presence of a pair of such invaginations in each segment 

 may no doubt be assumed as the primitive condition, there is variation 

 in this respect in the different groups. In the Lepidoptera, one 

 tracheal invagination appears in the pro-thorax, while none is found 

 in either the meso- or the meta-thorax. The embryos of most 

 Coleoptera and Hymenoptera (Apis, Butschli, Hylotoma, Graber, 

 No. 30), on the contrary, have no tracheal rudiment in the pro- 

 thorax, but possess such a structure on both the meso- and meta- 

 thorax. The same is the case in the embryo of Mantis (Graber, 

 No. 30). 



The tracheal invaginations as a rule develop only after the appearance of the 

 limb -rudiments. An exception to this rule is afforded by Apis, in which 

 the tracheal invaginations appear in the thoracic region before the belated limb- 

 rudiments. As a rule the invaginations appear almost simultaneously, only 

 rarely is there any indication of the order of development from before backward. 

 In H>/dro2)hihis, for instance, the meso-thoracic stigma appears somewhat earlier 

 than the stigmata of the other segments (Gkaber, No. 25). 



In the Coleoptera, structures conjectured by Heider (No. 38) and Wheeler 

 (No 95) to be the vestiges of tracheal invaginations have been observed on the 

 ninth and tenth abdominal segments. 



It should be mentioned here that certain ectodermal invaginations appearing 

 in the head have been regarded as tracheal formations which have lost their 

 primitive function and become secondarily modified. Carriers (No. 13) follow- 

 ing Moseley and Palmen (No. 161) has thus regarded the salivary glands and 

 the tentorial invaginations as modified tracheae. Others (Butschli, Geassi) 

 have considered the Malpighian vessels to be of the same type as the tracheal 

 invaginations. We shall further on give our reasons for not adopting this view. 



