TRICHOPTERA AND LEPIDOPTERA 339 



Since segmentation of the nucleus begins before it reaches the center 

 of the egg, the cleavage cells are located in the upper half of the egg at 

 the fourth hour after deposition. Segmentation beginning in the third 

 hour now goes on rapidly, and by the eighth hour the cleavage cells, 

 dividing mitotically, have nearly reached the surface, a few remaining 

 behind in the yolk to form the yolk cells. Of the cells penetrating the 

 periplasm to form the blastoderm, the cells destined to form the serosa 

 reach the surface a little earlier than those which are to form the embry- 

 onic rudiment. At this time no sharply marked cell walls can be dis- 

 tinguished. The actively dividing cells soon cover the entire surface. 



ser 



sp 

 A B 



Fig. 295. — Diacrisia. A, fragment of blastoderm as seen from within at junction of 

 germ band (gb) and serosa (ser). B, section of egg at the time of first maturation division. 

 (nu) Nucleus, (sp) Sperm. («/) Yolk. 



and by the tenth hour the blastoderm is completed with cell walls dis- 

 tinctly marked. The axis of the spindle of the dividing cells is invariably 

 parallel to the surface of the egg. 



After they reach the surface, the cells at the micropylar end and at 

 the lower pole of the egg no longer divide, are larger than the others, and 

 have two to four nuclei in each; the cells around the sides of the egg in 

 active mitosis are uninucleate. The former will form the serosa; the 

 latter, the germ band. It should be noted that Ganin (1869), Hatschek 

 (1877), and others long ago figured this condition for the Lepidoptera. 

 Harold (1815) also recognized and figured the serosa, although he did not 

 understand its significance. Kowalewsky (1871) stated that the cells 

 that form the germ band in Pterophorus are smaller and flatter than those 

 which form the serosa. Wood worth (1889) likewise noted that in 

 Euvanessa the transition between the germ band and the serosa is quite 

 abrupt. A surface view at the junction of germ band and serosa shows 



