326 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



The separation does not take place along the entire margin of the lateral 

 plates simultaneously but occurs first at the cephalic end of the egg as a 

 cap-like fold which grows rapidly caudad. A similar but slighter fold 

 is later formed at the caudal end of the egg. The two folds eventually 

 meet and coalesce near the caudal pole of the egg. At first the serosa 

 consists of the dorsal part of the blastoderm except for the median 

 longitudinal strip, but by the widening of the serosal bands the cells of 

 their inner margins creep up over the dorsal strip, and the latter becomes 



submerged in the yolk (Fig. 280). 

 Subsequently there is a fusion of 

 the serosal bands along the dorsal 

 mid-line of the egg, beginning first 

 at the cephalic end. While the 

 serosa is thus covering the dorsal 

 side of the yolk, it starts also to 

 cover the ventral side. The serosa 

 together with the anterior end of 

 the germ band then separates from 

 the yolk. Soon afterward it 

 separates from the yolk over the 

 entire cephalic pole of the egg, 

 rising up in the shape of a hemi- 

 spherical cap. Next it severs its 

 connection with the germ band 

 around the anterior end of the 

 latter and slides over it in the form 

 of a hood, thus forming the cephalic 

 fold (Fig. 283.4). This separation 

 of the serosa from the germ band 

 progresses caudad along its lateral margins, accompanied by the caudal 

 extension of the cephalic fold over the ventral face of the germ band. 

 When the cephalic fold has covered about one-half the ventral face of the 

 embryo, a caudal serosal fold (Fig. 283B) formed like the head fold appears 

 at the extreme caudal end of the germ band. The two folds now approach 

 each other until they finally meet and fuse at the caudal end (Fig. 283C). 

 At this time the serosa also separates from the yolk on the dorsal side of 

 the egg and thus forms a complete envelope surrounding the embryo. 

 The extension of the serosa over the surface is due principally, if not 

 exclusively, to a mere extension or spreading out of the original cells 

 present at an earlier stage. Just before hatching, the serosa is broken up 

 by the movements of the young larva. In the literature dealing with the 

 development of the bee the serosa is termed the "amnion." The latter, 

 however, is wholly lacking. 



Fig. 282. — Apis. Median sagittal sec- 

 tions through the anterior mid-gut epitheHum 

 rudiment, (ect) Ectoderm, (mes) Mesoderm. 

 {mge) Mid-gut epithelium rudiment, (mge. a) 

 Surface area of rudiment, (scr) Serosa. 

 {From Nelson.) 



