174 F. X. WILLIAMS 



and is especially thick at the caudal extremity, where it is often 

 two cells in thickness. It merges gradually on each side into 

 the germ band. The latter has as yet scarcely reflexed its borders 

 towards the yolk-mass, but the gastrula, which is of an invagi- 

 nated type, has already closed its lumen or central cavity, al- 

 though it is still recognizable as a groove over most of the germ 

 band. The invaginated portion seems to consist altogether of 

 homogeneous mesoderm. 



The next stage observed wafe the beginning of metamerization. 

 Two small lobes (L, fig. 38) on each side of the median line 

 represent the labrum, a clear central space the mouth, while a 

 pair of subangular projections, F, the antennae. The latter 

 already bear indications of segmentation and constitute the pos- 

 terior portion of the cephalic lobes. All of the remaining seg- 

 ments have rather uniform proportions, and their appendages, 

 including the first abdominal, are inconspicuous swellings. The 

 amnion has become a thin sheet and remains such from now 

 on. By the dorsal flexure of the sides of the embryo, the point 

 of origin of the amnion has been moved towards the dorsum, 

 where it is sharply marked off from the thick ectoderm (com- 

 pare figs. 46 and 48). The serosa, which lines the interior 

 periphery of the egg, is, like the amnion, a flattened sheet, but 

 its cells are far larger and less attenuated than those which com- 

 pose the amnion (S and A, figs. 51 and 52). 



A slightly older embryo is shown in figure 39, in lateral view, 

 and in figure 40, straightened out, in ventral view. The body 

 is stouter, somewhat fusiform, and it ssegmentation is well 

 defined. The cephalic lobes are further subdivided, the paired 

 lobes of the labrum being quite large, the antennal rudiments 

 more elongate and cylindrical, and the mouth-parts more dis- 

 tinct, especially the first maxillae. The legs, LG, and the pleuro- 

 pods, I A, of the first abdominal segment are conspicuous enough, 

 though the remaining abdominal appendages are decidedly ob- 

 scure. The caudal end of the abdomen is bilobed. The embryo 

 is bent backwards nearly into a circle. Cross sections of this 

 stage (figs. 47 and 48) show much advance over that represented 

 in figure 46. The sides of the body are well reflexed and the 



