8 WHEELER. [Vol. VIII. 



formation in Xiphidiitm, is, nevertheless, peculiar in two 

 respects: first, the envelopes are so closely applied to the 

 germ-band that in surface view their advancing edges can 

 be detected only with difficulty, though they may be distinctly 

 seen in sections; second, the point of closure of the envelopes 

 is situated further forward on the head than in Blatta, Hydro- 

 phihts, DorypJiora, etc. This I infer from an embryo, which I 

 figure (Fig. 15. PI. II.) Here the cells and nuclei of the 

 amnion and serosa have become much larger than the cells 

 and nuclei of the embryo. The edges of the folds are 

 unusually distinct and enclose a circular space through which 

 the oral and praeoral regions are clearly visible. On the 

 median anterior edge of the head the amnion and serosa are 

 completely interrupted. In no other insects have I found the 

 envelopes lacking on the anterior edge of the head in so late 

 a stage. This fact is probably significant when taken in con- 

 nection with changes about to occur in front of the head. 



The wide procephalic lobes are succeeded by the strap-shaped 

 body In this a number of segments have made their appear- 

 ance. These are in order from before backwards : the mandib- 

 ular (ind. s), the first maxillary (7nx. j'), the second maxillary, 

 (mx. S-) the three thoracic (/. s'^p. s3), and the first abdominal 

 (a. s^). Further back lies a small segment which is incom- 

 pletely constricted off from the first abdominal and which I 

 take to be the proliferating terminal segment, or telson. The 

 seven segments depicted in the figure are undoubtedly de- 

 finitive segments. The manner of their appearance will be 

 clear from a glance at Fig. I. In A the ligulate part of the 

 germ-band is seen to be faintly constricted at its base into two 

 segments with indications of a third. In B, a slightly later 

 stage, four definitive postoral segments are present, but a 

 portion of the germ-band still remains unsegmented. This is, 

 however, soon broken up into segments and we reach the stage 

 in Fig. 15, PI. II. It will be observed that the embryos in 

 Fig. I are in many respects older than that in Fig. 15, PI. II. 

 The antennee have made their appearance and the amniose- 

 rosal fold has closed completely. These embryos prove 

 several points: — first, that the wave of metameric segmen- 



