No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 



tation passes from before backwards dividing the germ-band 

 into 7 or 8 segments; second, that these segments are the 

 definitive segments and not macrosomites, or complexes of 

 definitive segments ; and third, that there is considerable varia- 

 tion in the time when segmentation sets in. To these points 

 I may add a fourth : segmentation appears first in the ectoderm 

 and only somewhat later in the mesoderm. 



M^'-: '■■■-■ 



-wvx S*- 



<^%00^ 



Fig. I. 

 A and B. Isolated embryos of Xiphidium in successive stages of metameriza- 

 tion. ind., indusium ; pel., procephalic lobe ; st., stomodaeum ; at., antenna ; md. s., 

 mandibular segment ; vix. s^, first ; mx. j^ second, maxillary segment ; /. j', 

 prothoracic segment. 



The indusium (Fig. 15,/. ^.) is still only a rounded thickening 

 of the blastoderm. Its small deep cells are continuous through 

 a zone of larger cells with the relatively very large and flat 

 elements of the primitive cell-layer. Two broad and flat 

 commissures appear to connect the organ with the procephalic 

 lobes. Thus a small space containing a few larger cells is 

 enclosed between the indusium and the head of the embryo. 

 This space (j/), seen as a clear spot in surface view, lies at the 

 breach in the envelopes. In many embryos the indusium is 



