APPENDIX. 



70: 



Fig. 657. 



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

 layer in the Libellulida; enters, together with the "amnion," 

 into the formation of tlie yolk sac. 

 Melnikow remarks that " it appears 

 from these facts that the ditferences 

 which we see in the embryonal mem- 

 branes of insects, are in direct rela- 

 tion to the mode in wliich 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, tlie germ 

 appears as seen in Fig. Co7. We 

 now see the amnion (am) surround- 

 ing the yolk mass, and the visceral 

 membrane (db) within partially envel- 

 oping the embrj'O. The head (vk, procephalie lobes, or anten- 

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

 tubercles, whicli are the rudiments of the mandibles, maxillce, 

 and labium or second maxillae. Behind tlie mouth-parts arise 

 six long slender tubercles forming the rudimentar}' legs, while 

 the primitive streak rudely marks out the ventral walls of the 

 thorax and abdomen. Fig. 658 represents the head and mouth- 

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

 clypeus ; ant, the antenna? ; mad, the mandibles ; max^, tlie first 

 pair of maxillae, and max-, the second pair of maxillae, or 

 labium. Fig. 659 represents the mouth-parts of the same 

 insect a little farther advanced, with the jaws and labium 

 elongated and closely folded together. Fig. 660 represents the 

 same still farther advanced ; the mandibles are sharp, and 

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

 maxillaa (mac^^) and labium (maoir) are still large, while after- 

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



Embryo of louse. 



