2O THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 



body wall was closed dorsally by mesoderm no yolk cells were observed in the body 

 cavity, though they were present in the yolk. The early embryo of Mantis is a 

 mass of undifferentiated cells lying on the surface of the yolk like the primi- 

 tive cumulus of spiders. 



The amniotic folds and inner layer arise as in other insects described. At 

 later stages the amnion is incomplete, not covering the ventral surface of the embryo. 



The maxillae have additional lobes at the base of the main axis of the appendage. 



Korotneff (1) has made a study of the embryology of Gryllotalpa by means of 

 sections. An abstract of his work is given here. On comparing it with the results 

 arrived at from the study of the Orthopterous insects, mantis and grasshopper, 

 there appear to be some important points of difference which can hardly be due 

 entirely to the more complete developmental history obtained by Korotneff. 



In the earliest stage observed by Korotneff there were four amoebiform cells in 

 the yolk. On the multiplication of these cells and their migration to the surface, the 

 blastoderm is formed. A stage occurs in the formation of the blastoderm during 

 which there are no cells in the yolk. Yolk cells subseqiiently migrate from the 

 surface ; Korotneff states that before the primitive cells, are con verted into blastoderm 

 cells, they have for a time no nuclei at all. No such disappearance of niiclei was 

 observed by me in the Orthoptera studied, or in the blastoderm formation of other 

 insects. It might, however, have been overlooked. The granular ill-defined nuclei of 

 the undifferentiated cells appear, from observations made in Thyridopteryx, to 

 become less granular and more vesicular when these cells reach the surface. 



The mesoderm of Gryllotalpa, according to Korotneff, arises, not by the separa- 

 tion of a median ingrowth from the outer layer, but by delamination on each side of 

 the median groove. This is certainly not the mode of origin of this layer in mantis. 

 Its origin in different Orthopterous insects may differ, however, for Korotneff's 

 observations seem to have been carefully made. The embryonic membranes of 

 Gryllotalpa arise, as in mantis and other insects described, as folds of blastoderm on 

 each side of the embryo, which meet and unite over its middle line. The united 

 inner limbs of these folds or the true amnion folds rupture in late embryonic life and 

 are absorbed. 



(1) Korotneff. Die Embryologie der Gryllotalpa. Zeit. f. wiss. Zool. 1884. 



