644 



CLASS IV. IN SECT A. 



epithelium-like blastoderm is formed all over the egg. A few 

 of the cleavage nuclei remain in the yolk and together with the 

 protoplasm around them develop into vitellophags which help 

 to liquefy the yolk and render it capable of being assimilated. 

 Very soon a differentiation appears along the ventral surface. 

 The blastoderm cells there thicken to form the embryonic band 

 and along the median line some cells are invaginated to 

 form the "lower layer" which is destined to form ultimately 



am. 



FIG. 404. Two diagrammatic sagittal sections through an insect-embryo to illustrate the 

 development of the embryonic envelopes. In A the germ-band (k, k') is not completely 

 grown over by the amniotic fold. In B the amniotic folds have united and completely cover 

 the germ-band, a anterior, b posterior pole of the egg ; v ventral ; d dorsal ; a/ amniotic 

 fold ; ah amniotic cavity ; am amnion ; do food -yolk ; ec ectoderm ; k cephalic end of 

 the germ-band ; k' posterior end of the germ-band ; s part of the serosa derived from 

 the amniotic fold ; s' part of the serosa derived from the undifferentiated blastoderm ; u 

 lower layer. After Korschelt and Heider. 



both mesoderm and endoderm. The dorsal epithelium has in 

 the meantime become thinner and lies like a layer of flattened 

 cells on the yolk. The embryonic band is not exposed. It 

 may sink into the yolk in the manner indicated in Fig. 403 or 

 more usually it is concealed by the upgrowth of an amnion as is 

 shown in Fig. 404. The embryonic band at an early stage shows 

 traces of segmentation corresponding with the somites of the 



