346 GENERAL BIOLOGY 



larva, the nucleus of the egg of the insects divided and each such par- 

 ticle of nucleus became a complete new embryo. "Thus a mass or chain 

 of embryos is produced, lying in a common cyst, and developing as their 

 larval host develops. In this way over a hundred embryos may result 

 from a single egg. Marchal points out the analogy of this phenomenon 

 to the artificial polyembryomy that has been induced in Echinoderm 

 ( ) and other eggs by separating the blastomeres, 



and suggests that the abundant food-supply afforded by the host-larva 

 is favorable for this multiplication of embryos, which may be, in the first 

 instance, incited by the abnormal osmotic pressure on the egg." 



When many embryos develop from a single egg in the way just de- 

 scribed it is called polyembryony. 



H. H. Newman has shown that in the ant-eater, armadillo, in which 

 three to nine embryos commonly form different species, all develop from 

 a single egg. The fertilized egg does not split into separate parts but 

 evaginates in different portions to form separate embryos. 



ALTERNATION OF GENERATIONS 



A true alternation of generation has been found in Hymenopterous 

 gall flies (Fig. 226), in which a complete asexual generation (complete 

 from egg to adult) succeeds a complete sexual generation (egg to adult), 

 each generation being parasitic on a different host plant. The adults 

 in each case bear no resemblance to each other; in fact, they have not 

 only been described as different species, but actually as different genera. 



EMBRYOLOGY 



The flapping of wings or the "singing" of the male grasshopper 

 attracts the "unfertilized" females. The sperm are then injected into the 

 female receptacle, from whence they work their way into the various 

 eggs. 



The zygote thus formed, begins to segment mitotically, forming the 

 embryo on top of the yolk close to the egg-shell. There are two pro- 

 tective membranes, the innermost being known as an amnion 

 ( ) or chorion ( ), and the 



outer as the sero&a ( ). As soon as the embryo has 



used up the yolk as food it is ready to hatch. 



However, all of this process does not take place in the body of the 

 grasshopper. Soon after fertilization the female drills a hole in the 

 ground with the hard portions of the ovipositor and deposits the eggs 

 which are then covered. These hatch in the spring. It is here, in its 

 warm underground cage, that most of the development described above 

 takes place. 



"By opening and shutting the ovipositor a hole (Fig. 225), slightly 

 curved, is quickly drilled in the ground. This drilling process goes on 



