320 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



ami 



several clumps of embryonic cells, each clump surrounded by a nucleated 

 membrane, form a polygerm, the entire mass enclosed in the trophamnion 

 (Fig. 27-4). A lengthening of the polygerm follows. Ingrowths from the 

 trophamnion (polar membrane) eventually surround each primary mass 

 to become the outer envelope, or membrane, of each mass. The number 

 of embryonic cells included in each primary mass is extremely variable, 

 ranging from 4 or 5 to as many as 50 cells. Embryonic cells are con- 

 stantly dividing, so that a primary mass with few cells soon has that 

 number increased. Likewise, the number of primary masses within the 

 polygerm is variable, ranging from 15 to 20. 



In the completed polygerm each primary mass thus consists of several 

 embryonic cells surrounded by a relatively thick inner membrane, and 



the various primary masses are more or 

 less separated from one another by 

 ingrowths from the trophamnion. Soon 

 after the polygerm is formed, the pri- 

 mary masses begin to multiply by 

 fission. The division is initiated by a 

 constriction of the inner membrane, 

 followed by a corresponding constric- 

 tion or ingrowth of the trophamnion, 

 resulting in the formation of secondary 

 masses. A subsequent similar division 

 of the secondary masses results in the 

 formation of tertiary masses, the latter 

 in turn dividing several times to pro- 

 FiG. 274:.—Litomastix. Section of ^j^^g ^j^g definitive components each of 



young polygerm. (amt) Trophamnion. , . , , , , r •, 



(c) Embryonic cells, (csp) Spindle cells which becomes the embryo ol a parasite, 

 forming an inner membrane. ^he formation of the tertiaries may 



begin as early as the end of the fourth day and continue through the 

 sixth day. From the seventh to the tenth day the multiplication of the 

 tertiary masses and their components goes on with great rapidity, form- 

 ing a complex structure by the eleventh day. 



The components later may completely separate from each other, 

 becoming scattered throughout the body cavity of the host and forming 

 new centers of proliferation. The rate of their distribution to various 

 parts of the body cavity of the host to a very great extent depends upon 

 their relation to the host tissues. If the polygerm is imbedded in adipose 

 or other tissue, the scattering of the tertiary masses and their com- 

 ponents may be greatly delayed. On the other hand, if the polygerm 

 happens to lie free within the body cavity, the dispersal of its com- 

 ponents may begin very early, even as early as the primary stage. 

 Eggs of the parasite may be laid in any part of the host egg, but they dis- 



