THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 13 



The migratory mesoderm cells surrounding the digestive tract may have been 

 wrongly regarded by Hatscheck as endoderm since, as already stated, he seems to 

 have mistaken other migratory cells for endoderm. 



The closure of the dorsal surface of the body is interesting. The amniotic 

 folds (AM' Fig. XXV) grow dorsally more rapidly than the ectoderm of the body 

 walls. These opposite amniotic folds finally unite and the inner limb of each fold 

 forms a portion of the dorsal surface of the body. The outer limbs of the amniotic 

 folds unite to form a sack in which the entire embryo lies. This sack contains no 

 yolk, but is apparently filled with fluid. Just before the union of the opposite amni- 

 otic folds, there is formed what may be termed a dorsal organ (Fig. XXVI,) though 

 it does not correspond to the dorsal organ described for some other insects. 1 



A study of figures XV, XXIV, XXV, XXVI, will make clearer what has been 

 said concerning the origin of the endoderm and the union of the amniotic folds on 

 the dorsal surface of the body. 



Figure XV has already been described. It represents a transverse section of a 

 well advanced embryo. The tracheal invagiuations are shown in the figure (TR) to be 

 iuvaginations of ectoderm. The appendages are outgrowths of the body cavity 

 occurring between the tracheal invaginatious and the nerve ganglia. 



On comparing figure XV with figure XI or with preceediug figures it will be 

 seen that in the former, which represents a transverse section of a more advanced 

 embryo than the latter, the amniotic folds have extended farther towards the dorsal 

 surface of the figure. Figures XXIV, XXV, XXVI, are drawings of an older 

 embryo than that represented by figure XV. Here it will be seen that the inner 

 portions of the amniotic folds (AM' in Figs.) are growing together and forming the 

 dorsal wall of the body as described. 



Figure XXVI represents a section of an advanced embryo just before the union 

 of corresponding portions of the amniotic folds of opposite sides, when the outer 

 and inner portions of each fold are united by intermediate amiiiou which forms a 

 sort or dorsal organ. 



Inasmuch as no yolk lies between the embryo and the amnion it will readily be 



01 Brandt. Beitrage zur Entvvicklungsgeschichte d. Libellula. 



