EMBRYOLOGY 



151 



embryology of another Orthopteron, considers it will prove 

 to be incorrect. When the secondary segmentation occurs the 

 anterior of the two cephalic divisions remains intact, while the 

 second divides into the three parts that afterwards bear the 

 mouth parts as appendages. The thoracic mass subsequently 

 segments into three parts, and still later the hind part of the 

 ventral plate undergoes a similar differentiation so as to form the 

 abdominal segments ; what the exact number of these may be 

 is, however, by no means easy to decide, the division being but 

 vague, especially posteriorly, and not occurring all at once, but 

 progressing from before backwards. 



The investigations that have been made in reference to the 

 segmentation of the ventral plate do not at present justify us 

 in asserting that all Insects are formed from the same number 

 of embryonic segments. The matter is summarised by Lowne, to 

 the effect that posterior to the procephalic lobes there are three 

 head segments and three thoracic segments, and a number of 

 abdominal segments, " rarely less than nine or more than eleven." 

 It will be seen by referring to Figure 81 that the segmentation 

 appears, not simultaneously, but 

 progressively from the head back- 

 wards ; this of course greatly in- 

 creases the difficulty of determin- 

 ing by means of a section the real 

 number of segments. 



The later stages in the develop- 

 ment of Insects are already proved 

 to be so various that it would be 

 impossible to attempt to follow 

 them in detail ; but in Fig. 8 2 

 we represent a median section 

 of the embryo of Zygaena filipeii- 

 dula at the fifth day. It shows 

 well some of the more important 

 of the general features of the de- 

 velopment at a stage subsequent to 

 those represented in Fig. 81, A, B, 

 C. The very distinct stomodaeum 



{st) and proctodaeum (j)r) are seen as inflexions of the external 

 wall of the body ; the segmentation and the development of the 



tic ^'r^\ aJ 



Frr.. 82. Embryo of a moth {Zygaena) 

 at the fifth day (after Graber) : am, 

 amnion ; s, serosa ; j), procephalic 

 lobes ; st, stomodaeum ; pr, procto- 



daeum 



9, 9', 



the mouth parts 

 or head ai:)pendages ; th}, th^, th^, 

 appendages of the thoracic segments ; 

 a}-a}^, abdominal segments ; s.g, sali- 

 vary gland. 



