302 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



in the head that later fuse to form the labrum, but these lobes are not 

 present in Brachyrhinus. As segmentation 

 progresses toward the posterior end, the 

 appendages of the head and thorax become 

 more pronounced, and the embryo begins to 

 reduce its length (Fig. 245). This occurs 

 on the fourth day. 



The gastrular furrow which is completed 

 on the third day marks externally the 

 invaginating ridge of cells that will become 

 the inner layer (Fig. 246) . The inner layer 

 becomes arranged in transverse segmented 

 masses when segmentation occurs; and 

 when the neural groove appears, these 

 masses are further divided along the center 

 line. It is probable that at this time cells 

 that will become blood cells are liberated 

 along the middle strand. The inner layer 

 soon separates, irregularly at first, into 



two layers, and on the fourth day at the lateral extremities of each 



Fig. 245, 



Fourth- 





gasir.fr 



am 

 Fig. 246. — Brachyrhinus. Cross section through abdomen. Lower-layer formation. 

 {am) Amnion, {ect) Ectoderm, {gastr. fr) Gastrular furrow, {il) Inner layer. {,ser) 

 Serosa, {sub. ser) Subserosa. 



segmental mass of the inner layer the coelomic sacs appear (Fig. 247, 



