CHAPTER Vlir. 



ACANTHOCEPHALI. 



The eggs of the Acanthocephali are detaclied from the 

 ovarium as membraneless, usually spindle-shaped cells, and 

 then come to lie in the interior of the body of the female. 

 Here they are fertilized, after which each egg is surrounded 

 by a delicate transparent membrane, and then begins to 

 cleave. When this (in EcMnorhynchus gigas) has advanced 

 as far as the formation of twelve blastomeres, a second mem- 

 brane is formed under the first, which has separated some 

 distance from the Qgg, and to which are added in the course 

 of the development two more protective envelopes, so that 

 finally four of them are present. This applies to Echino- 

 rhynchus gigas (Fig. 118 A). Ordi- 

 narily three such embryonal mem- 

 branes are formed, the middle one of 

 which acquires a considerable thick- 

 ness and firmness by the deposition 

 of concretions of a brownish colour. 

 These structures are particularly 

 noteworthy, for the reason that they 

 first make their appearance during 

 cleavage, and therefore are not to be 

 looked upon as egg-membranes, but 

 as a kind of embryonal membrane ; 

 still they do not appear to have any cellular structure. 

 They recall the embryonal membranes occurring in the 

 Tgeniada?, which may also acquire a considerable firmness. 



During the formation of the embryonal membranes the 

 cleavage has continued.^ It is unequal, and, according to 



* In this connection we follow, in addition to the older observations of 

 Greeff and especially Leuckart, the newer investigations of Kaiser on 



249 



A-'^< 



Fig. 112. — 4 to D, four 

 cleavage stages of EcMnor- 

 hynchus protens (after Leuc- 

 kakt) ; eh, first embryonal 

 membrane. 



