646 



CLASS IV. INSECTA. 



and ultimately in the dorsal. By this time the yolk has come 

 to lie completely within the alimentary canal. In some insects 

 however the above account does not hold good. In them the 



alimentary canal seems to 

 arise entirely from the ecto- 

 dermal invaginations of the 

 stomodaeum and procto- 

 daeum.* 



The salivary glands arise 

 as ectodermal invaginations 

 and originally open on the 

 exterior of the body but 

 subsequently into the stomo- 

 daeum. The malpighian 

 tubules arise as out-growths 

 of the proctodaeum. In 

 one or two genera, e.g. Apis 

 they appear before the proc- 

 todaeum is formed and thus 

 for a time open on the 

 exterior. 



What is left of the " lower- 

 layer," after the separation 

 of the endoderm rudiments 

 before and behind, forms 

 two bands, and segmental 

 coelomic cavities appear 

 along each band. These 

 extend into the limbs and 

 are better developed in the 

 Orthoptera than in other 

 groups of Insects. The hae- 

 mocoel of the Insecta has 

 nothing to do with these 

 coelomic cavities but is 

 formed by the separating of 

 the embryonic band from the yolk and for a time is bounded 

 by the yolk on one side and certain sparsely scattered mesen- 



FIG. 406. Transverse section through the ab- 

 dominal region of an embryo of Phyllodromia 

 gernMnica, after the circumcrescence of the yolk 

 and the formation of the dorsal surface are 

 completed, bg ventral ganglionic chain : cz 

 . rudiment of the efferent genital duct ; d food- 

 yolk ; el terminal filament ; en entoderm ; fk 

 tissue of the fat-body ; gz genital cells ; h 

 heart ; ps pericardial septum ; s tracheal stig- 

 ma ; sp splanchnic mesoderm ; vm ventral 

 longitudinal muscle. 



* Heymons, SB. Ak. Berlin, 1894, and SB. Ges. Naturf. Berlin, 1896 ; 

 and Witlaczil, Zeit. wiss ZooL, xl, 1884, p. 559. 



