POLYEMBRYONY 



355 



The cytoplasm around the nucleus then becomes separated off from 

 the rest of the egg, which contains the polar nuclei, and conse- 

 quently the egg becomes divisible into an embryonic area and a 

 polar area (Fig. 89, B). A little later, the embryonic nuclei are 

 seen in division, and the 



. -S— P, 



polar nuclei have given rise 

 to the paranucleus or nucleus 

 of the future trophamnion 

 (Fig. 91). The developing 

 egg then soon becomes 

 enclosed in a cellular adven- 

 titious sheath derived from 

 the surrounding tissues of 

 the host ; the embryonic 

 nuclei increase in number, 

 the paranucleus also under- 

 goes division, and the original 

 polar area comes to form a 

 definite covering, or troph- 

 amnion, surrounding the 

 whole of the embryonic area 

 of the egg (Fig. 91). The 

 first indication of poly- 

 embryony is the division of 

 the embryonic area into 

 small groups of cells, or 

 morulse, which are seen to 

 lie in cavities within the 

 trophamnion. The whole egg 

 increases in size, and mean- 

 time both the adventitious 





04^ 



c> 



s 



g 



e- 

 B 



tr 





Fig. 90. Encyrtus fuscicollis, early poly- 

 embryonic development. A, Egg with 

 first and second polar bodies p^, p^. 

 B, Division into embryonic area e and 

 trophamnion tr. C, Transverse section 

 of polygerminal mass, a, Adventitious 

 sheath. e, e. Embryos, g, Germ-cell 

 determinant. n, Oocyte nucleus. 



p. Paranucleus. tr, Trophamnion. 



, . , J . 1 • (Adapted from Martin.) 



sheath and trophamnion ^ 



grow simultaneously and maintain a complete double investment 



(Fig. 90, C). The further development of the morulas need not 



concern us here, and it will suffice to add that each subsequently 



becomes a separate embryo, destined to transform into an 



individual parasite. The number of embryos which originate 



12—3 



