342 INSECT A. 



The development of the tracheae strongly supports the view, 

 arrived at by Moseley from his investigations on Peripatus, that they 

 are modifications of cutaneous glands. 



The salivary and spinning glands are epiblastic structures, which in 

 their mode of development are very similar to the trachea?, and perhaps 

 have a similar origin. The salivary glands arise as paired epiblastic 

 invaginations, not, as might be expected, of the stomodgeum, but of the 

 ventral plate behind the mouth on the inner side of the mandibles. 

 At first independent, they eventually unite in a common duct, 

 which falls into the mouth. The spinning glands arise on the inner 

 side of the second pair of maxillae in Apis and Lepidoptera, and form 

 elongated glands extending through nearly the whole length of the 

 body. They are very similar in their structure and development to 

 salivary glands, and are only employed during larval life. They no 

 doubt resemble the mucous glands of the oral papillae of Peripatus, 

 with which they have been compared by Moseley. The mucous 

 glands of Peripatus may perhaps be the homologous organs of the 

 first pair of maxillae, for the existence of which there appears to be 

 some evidence amongst Insects. 



Mesoblast. It has been stated that the mesoblast becomes divided 

 in the region of the body into two lateral bands (fig. 189 A). These 

 bands in many, if not all forms, become divided into a series of 

 somites corresponding with the segments of the body. In each of 

 them a cavity appears the commencing perivisceral cavity which 

 divides them into a somatic plate in contact with the epiblast, and a 

 splanchnic plate in contact with the hypoblast (fig. 189). In the 

 interspaces between the segments the mesoblast is continuous across 

 the median ventral line. The mesoblast is prolonged into each of the 

 appendages as these are formed, and in the appendages there is 

 present a central cavity. By Metsclmikoff these cavities are stated to 

 be continuous, as in Myriapods and Arachnida, with those of the 

 somites ; but by Hatschek (No. 414) they are stated to be independent 

 of those in the somites and to be open to the yolk. 



The further details of the history of the mesoblast are very imperfectly 

 known, and the fullest account we have is that by Dohrn. (No. 408) for 

 Gryllotalpa. It would appear that the mesoblast grows round and encloses 

 the dorsal side of the yolk earlier than the epiblast. In Gryllotalpa it 

 forms a pulsating membrane. As the epiblast extends dorsalwards the 

 median dorsal part of the membrane is constricted off as a tube which 

 forms the heart. At the same time the free space between the pulsating 

 membrane and the yolk is obliterated, but transverse passages are left at 

 the lines between the somites, through which the blood passes from the 

 ventral part of the body to corresponding openings in the wall of the 

 heart. The greater part of tlie membrane gives rise to the muscles of the 

 trunk. 



Ventrally the mesoblastic bands soon meet across the median line. 

 The cavities in the appendages become obliterated and their mesoblastic 



