42 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



head sacs is found in the higher arthropods indicate that the formation of the 

 cavities in the cephaUc mesoderm is a temporary accompaniment of advancing 

 organization in the prostomial lobe, but the temporary nature of the head 

 cavities might equally suggest that they are purely ontogenetic structures as 

 claimed by Faussek, for coelomic cavities in general. 



Snodgrass says further: 



The principal reasons for regarding the oculo-antennal region of the arthropod 

 head, here defined as the acron, as representing a primarily unsegmented archi- 

 cephalon corresponding with the annelid prostomium may be summarized as 

 follows: (1) There is never any external division of the acronal region into seg- 

 mental areas; (2) there is no specific evidence of the cephalization of primarily 

 postoral somites, except in the case of the tritocerebral somite; (3) the embryonic 

 coelomic sacs of the first antennae, the preantennae, and the labrum are formed 

 directly where they occur in the cephalic mesoderm and give no evidence of 

 having been drawn forward from behind the mouth; (4) coelomic sacs of the 

 acronal region, so far as known, are best developed in the higher arthropods and 

 thus do not appear to be primitive structures; (5) the protocerebral and deuto- 

 cerebral parts of the brain are always connected by preoral commissures, the 

 only postoral cerebral commissure being that of the cephalized tritocerebral 

 ganglia; (6) the mouth and labrum are innervated from the tritocerebral ganglion, 

 which would not likely be the case if several other postoral ganglia preceded the 

 tritocerebral ganglia; (7) paired appendages, sense organs, and primarily discrete 

 nerve centers pertain both to the annelid prostomium and to the arthropod 

 acron; (8) the first antennae of the arthropods never have the structure or 

 musculature of the following appendages; in the Crustacea they are never truly 

 biramous. 



Snodgrass (1938) gives a brief review of the facts now known concern- 

 ing the development of the procephalic mesoderm and nervous system of 

 the arthropods wherein he maintains that the facts are not inconsistent 

 with the idea that both the coelomic sacs and the multiple nerve centers 

 may be formed directly in the otherwise unsegmented acronal region and 

 that the phenomena of embryonic development pertaining to the head are 

 most easily understood if they are taken approximately at their face 

 value for phylogenetic recapitulations. 



The theory proposed by Snodgrass (1938) assumes that the archi- 

 cephalic nervous system of the arthropods, as that of the annelids, has 

 been built up from groups of ganglionic cells centering upon a fibrous 

 commissural tract arched forward around the mouth and continuous 

 posteriorly with the ventral nerve cords of the somatic system (Fig. 30). 

 The primary cephalic ganglion included a median anterior ganglion, 

 paired protocerebral and optic ganglia, paired preantennal ganglia, and 

 paired first antennal ganglia. " That these ganglia belong to the preoral 

 acron (acr) is shown by the fact that the paired ganglia are always con- 



