POLY EM BRYONY 



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the host tissues, which they wholly consume. In eggs which 

 develop monoembryonically, the 4-celled stage gives rise to a 

 single blastula, but otherwise the development is similar in the 

 two cases. We have in P. hiemalis the simplest possible type of 

 polyembryony, and it appears that it has arisen from an already 

 specialised method of monoembryonic development, which is also 

 found in other Platygasterids. In another species, P. vernalis, 

 which is also parasitic upon the Hessian fly, these same observers 



Fig. 91. Platygaster hiemalis, early polyembryonic development. 

 A, Egg with four embryonic nuclei. J5, Division into two embryos 

 surrounded by a common trophamnion. C, Transverse section of 

 an embryo in early blastula or 16-celled stage, of which seven are 

 shown. (Adapted from Leiby and Hill.) Lettering as in Fig. 90. 



state that it lays a single egg in the egg of its host. The parasitic 

 egg is deposited in such a manner that it eventually comes to lie 

 in the mid-intestine of the host larva and consequently does not 

 become invested by an adventitious sheath. During early 

 development the embryonic nucleus undergoes division until 

 16 nuclei are produced. A cell membrane, enclosing a small 

 amount of cytoplasm, becomes formed around each nucleus, and 

 the cell thus constituted is recognised as a germ. Each germ maj" 

 be the progenitor of either one or two larvae, depending upon 



