APPENDIX. 



707 



Fig. 657. 



St. Petersburg Academy, 1869) also shows that the visceral 

 layer in the Libelhilidaj enters, together with tlie "amnion," 



into the formation of the yolk sac. 

 Melnikow remarks that " it appears 

 from these facts that the differences 

 which we see in the embryonal mem- 

 ' brancs of insects, are in direct rela- 

 tion to the mode in which the prim- 

 itive band is formed. It seems, 

 therefore, that the mode of origin of 

 the primitive band, or its position in 

 relation to the yolk, is concerned in 

 the above mentioned differences of 

 the embryonal membranes. 



Development of the Louse. — 

 After the budding out of the limbs 

 from the primitive band, the germ 

 appears as seen in Fig. 057. We 

 now see the amnion (am) surround- 

 ing the yolk mass, and the visceral 

 membrane (db) within partially envel- 

 oping the embryo. The head (wA:, procephalic hjbes, or anten- 

 nal segment,) besides the antennie (as), bears three pairs of short 

 tubercles, which are the rudiments of the mandibles, maxillie, 

 and labium or second maxilhe. Behind the mouth-parts arise 

 six long slender tubercles forming the rudimentary legs, while 

 the primitive streak rudely marks out the ventral walls of the 

 thorax and abdomen. Fig. f.58 represents the head and mouth- 

 parts of the embryo of the same louse ; vk is tiie forehead, or 

 clypeus ; an^ the antennre ; mad, the mandibles ; viax\ the first 

 pair of maxilla?, and inax", the second pair of maxilUie, or 

 labium. Fig. 059 represents the mouth-parts of the same 

 insect a little farther advanced, with the jaws and labium 

 elougated and closely folded together. Fig. GOO represents the 

 same still farther advanced ; the mandibles are sharp, and 

 resemble the jaws of the Mallophaga or biting lice ; and the 

 maxillae (max^) and labium (viux-) are still large, while after- 

 wards the labium becomes nearly obsolete. Fig. 661 repre- 



Embryo of louse. 



