TRACHEATA. 379 



the Tracheata, a large part of the mesoblast is not derived from the 

 mesoblastic plate, but is secondarily added from the yolk-cells. 



In all Tracheata the yolk-cells give rise to the mesenteron which, 

 in opposition, as will hereafter appear, to the mesenteron of the Crus- 

 tacea, forms the main section of the permanent alimentary tract. 



One of the points which is still most obscure in connection with 

 the embryology of the Tracheata is the origin of the embryonic mem- 

 branes. Amongst Insects, with the exception of the Thysanura, such 

 membranes are well developed. In the other groups definite mem- 

 branes like those of Insects are never found, but in the Scorpion a 

 cellular envelope appears to be formed round the embryo from the 

 cells of the blastoderm, and more or less similar structures have been 

 described in some Myriapods (vide p. 322). These structures no doubt 

 further require investigation, but may provisionally be regarded as 

 homologous with the amnion and serous membrane of Insects. In 

 the present state of our knowledge it does not seem easy to give any 

 explanation of the origin of these membranes, but they may be in 

 some way derived from an early ecdysis. 



