THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 15 



including the last somite or the so called post abdomen. The number of somites is 

 not well shown in figure XXXIV, which is not strictly median. The last or eleventh 

 abdominal somite is covered dorsally and corresponds to the last somite of Thyridop- 

 teryx. According to Packard, the last somite, or post abdomen, consists in Diplax, n> 

 of several somites. 



This segmentation of the post abdomen was not observed in Ch^sopa through 

 the observations made on this insect were confessedly very incomplete. 



COLEOPTERA. 



The embryology of Meloe, the parasitic beetle, was studied as representing the 

 Coleoptera. The earliest stage obtained showed a surface blastoderm enclosing 

 central cells (Fig. XXXV). Later the blastoderm cells become more columnar, 

 (Fig. XXXV) and there are apparently no cells in the yolk; though unless a com- 

 plete series of sections of a very well preserved egg be obtained, it is impossible to 

 be positive on the latter part. Apparently, however, a stage occurs in which there 

 are no cells in the yolk, consequently the yolk cells, numerous in latter stages, 

 probably arise from the blastoderm or from the embryo. The earliest embryo (Fig. 

 XXXVI) showed, in cross sections, incipient amniotic folds on each side with a 

 median blastopore. 



It becomes separated from the surface on the union of the amniotic folds, but 

 does not sink into the centre of the egg as does the embryo of Thyridopteryx, (Figs. 

 XXXVII, XXXVIII). 



Amcebiform cells (YC' in Figs.) can be seen on the dorsal or yolk side of the 

 embryo at this stage. These are probably yolk cells which arise by division from 

 the embryo and subsequently migrate into the yolk. The median invagination be- 

 comes constricted of and forms the inner layer, (Fig. XXXIX). 



No observations were made on the origin of the nervous system or on the origin 

 of the endoderm. The nerve ganglia become separated from the surface ectoderm, 

 and in advanced embryos all trace of the blastopore has disappeared as in Thyri- 

 dopteryx (Fig. XL). 



(!) Packard. Guide to the study of Insects pp. 56-57. 



