ISO 



EMBRYOLOGY 



of division or arrangement of the body into segments appears 

 very early, as shown in our Figure of some of the early stages 

 of development oi Una (a beetle), Fig. 81. In A the segmen- 

 tation of the ectoderm has not commenced, but the procephalic 

 lobes {P (J) are seen ; in B the three head segments are distinct, 

 while in C the thoracic segmentation has occurred, and that 

 of the abdomen has commenced. Graber considers that in this 

 species the abdomen consists of ten segmental lobes, and a 

 terminal piece or telson. According to G-raber ^ this is not a 

 primitive condition, but is preceded by a division into three or 



Fig. 81. — Early stages of the segmentation of a beetle {Lino) : A, segmentation not visible, 

 1 day ; B, segmentation of bead visible ; C, segmentation still more advanced, 2J 

 days ; PC, procephalic lobes ; g^, g-, g^, segments bearing appendages of the head ; 

 ih, thorax ; tli}, th?, tli-', segments of the thorax ; a}, C(?, anterior abdominal. 



four parts, corresponding with the divisions that will afterwards 

 be head, thorax, and abdomen. This primary segmentation, he 

 says, takes place in the Hypoblast (Endoderm) layer of the ventral 

 plate ; this layer being, in an early stage of the development of 

 a common grasshopper {Stenohothrus variaMlis), divided into 

 four sections, two of which go to form the head, while the others 

 become thorax and abdomen respectively. In Zina the primary 

 segmentation is, Graber says, into three instead of four parts. 

 Graber's opinion on the primary segmentation does not appear 

 to be generally accepted, and Wheeler, who has studied ' the 



1 Morph. Jalirb. xiv. 1888, p. 345. - J. 3Iorphol. viii. 1893, p. 1. 



