360 PARASITISM 



hence the term trophamnion. In the eggs just alluded to the 

 chorion is either wanting, or is shed at an early stage. The 

 type of membrane which, as already described, originates from 

 the polar area of the egg occurs in poly embryonic forms 

 irrespective of their taxonomic affinities, and also in certain 

 closely related monoembryonic species. In other examples an 

 embryonic membrane is described as originating either as the result 

 of simple delamination of the blastoderm, or by the extrusion of 

 cellular material to the exterior, and its ultimate fusion to form a 

 complete investment of the egg. The process of delamination is 

 described by Henneguy (1892) in Chalcis (Smicra) clavipes and 

 by Silvestri (1908) in Encyrtus aphidivorus and Oopthora semblidis, 

 these three species being Chalcids. Spencer (1926) states that in 

 the Braconid Diccretus rapce the embryonic membrane is formed by 

 the delamination or extrusion of cells from the ectoderm early in 

 development. A unique condition is described by Silvestri (1906) 

 in Litomastix truncatellus in which two embryonic membranes are 

 present, viz. an outer membrane or true trophamnion, and an 

 inner membrane formed by delamination during the morula stage 

 of development. It is evident that the actual method of origin 

 of the second type of envelope needs fuller investigation, but it is 

 probable that functionally both types behave alike and their 

 subsequent history is very much the same. The term trophamnion 

 is best confined to a membrane derived from the polar bodies of 

 the egg, while in those cases where the fate of the polar bodies is 

 different, and the embryonic membrane is formed by delamination 

 or by cellular extrusion, such a membrane may be termed a 

 pseudoserosa. The use of the terms amnion or serosa is open to 

 objection, since these envelopes have a different origin and 

 significance. 



Either just before, or a short time after, the larva has assumed 

 independent existence the embryonic membrane (either troph- 

 amnion or pseudoserosa) usually distintegrates ; its cells separate 

 apart or adhere in small aggregations and become liberated into 

 the body-cavity of the host. Here they may continue to grow 

 in size, but undergo evident nuclear changes, and it is probable 

 that they usually serve as food for the growing parasitic larva. 



