GENERAL CHARACTERS OF THE EXO-SKELETON. 103 



band of the embryo in the ventral region and the raphe in the 

 dorsal region, formed by the union of the two lateral halves 

 of the somatopleure on the dorsal surface of the yelk. 



Primary Sutures are the remains of the inflections of the 

 epiblast between the primary somites of the embryo. 



Secondary Sutures are either symphyses or syndesmoses, 

 separating the sclerites of a metamere. 



Morphological Considerations. -The median dorsal sutures are 

 the first to disappear in the more highly-modified types. In 

 the higher forms of the Insecta the median sutures are 

 rarely present, but their position is indicated by complex 

 inflections of the integument in the ventral region. Such are 

 the entothoracic and endocranial processes which support the 

 ganglia of the ventral chain. The consolidation of the somites, 

 which is characteristic of the most highly-modified types, 

 leads to the obliteration, or partial obliteration, of the primary 

 sutures, whilst the secondary sutures become larger and more 

 numerous, and frequently form important syndesmoses. These 

 are especially concerned in the mechanism of flight and the 

 respiratory movements of the thorax. Secondary sutures also 

 often form strongly-marked internal ridges and processes, 

 which afford attachment to muscles and add greatly to the 

 solidity of the external skeleton ; thus the secondary sutures 

 appear to take the place of the primary sutures, with advanced 

 complexity. The more highly - developed a part, the more 

 numerous and important are its secondary sutures, and the 

 greater the tendency to the consolidation of several somites, 

 with the consequent obliteration of the primary sutures between 

 them. These laws are exemplified in the structure of the head 

 and thorax, and the confusion which exists in the nomenclature 

 of the sclerites of the cephalic and thoracic skeleton depends 

 mainly on the fact that they have hitherto remained unrecog- 

 nised. 



Although the secondary sutures are very similar in corre- 

 sponding somites of widely-divergent types of insect, their 

 disposition and number have possibly only an indirect mor- 

 phological significance, and result from similar adaptive 



