TRACHEATA. 413 



arise as paired epiblastic invaginations. not, as might be 

 expected, of the Stomodaeum, but of the ventral plate behind 

 the mouth on the inner side of the mandibles. At first indepen- 

 dent, 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 deve- 

 lopment 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 Metschnikoff 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 the membrane gives rise to the muscles of the trunk. 



