THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 17 



The earliest trace of the embryo was obtained from sections of the eggs of the 

 Mantis. At this early stage the embryo is a mass of undifferentiated cells lying on 

 the surface of the egg. (Fig. XLJ, E.) The blastoderm is not formed at this stage. 

 Some cells remain in the yolk; whether they are all at the surface at some subse- 

 quent stage could not be determined. 



Figures XLJI & XLJII, show two stages in the formation of the amniotic 

 folds of Mantis. It will be seen that the amnion, as seen in cross sections, 

 arises on each side of the embryo as folds of blastoderm, which meet and unite 

 over its middle line. 



When the union of the folds is effected, and the embryo is separated from the 

 surface and covered veiitrally by the amnion, the inner layer is formed, as in Meloe 

 and Thyridopteryx, as an ingrowth from the median line of the embryo. Whether 

 the inner layer gives rise to both mesoderm and endoderm could not be determined 

 in the case of Mantis as no advanced embryos of this insect were studied. Figure 

 XLIV is a drawing of a transverse section of an early Mantis embryo, showing the 

 origin of the inner layer ' ' from the median groove. From a study of more 

 advanced grasshopper embryos, however, it seems probable that the yolk cells do not 

 take part in the formation of the endoderm ; consequently that layer is formed in 

 the grasshopper as in Thyridopteryx from the inner germ layer. Ayers u/) in his 

 memories on CEcanthus, an Orthopterous insect, described undifferentiated cells 

 which never reach the surface to form blastoderm but remain in the centre of the 

 egg as yolk cells. Unlike the corresponding cells of Thyridopteryx, these cells, 

 according to Ayers, take part in the formation of the endoderm. The serosa or 

 outer embryonic membrane is taken into the body cavity of the embryo through its 

 enclosed dorsal surface, and also forms parts of the endoderm. Nothing at all cor- 

 responding to such an absorption of embryonic membranes was observed in the 

 advanced grasshopper embryo. This embryo is perhaps best understood by following 

 the series of cross sections represented by figures XLVII-LXV. Most of the sec- 

 tions figured were selected from a single series from an advanced embryo. They 

 were drawn in order, from before backwards. Figure XLVII, represents a cross 

 section of the head of an advanced grasshopper embryo. Reference to the meaning 



(*> Ayers. .Memoirs of the Boston Society of Natural History, Vol. Ill no. VIII. 



