parasitic on Freshioater Mussels. 57 



say witli certainty, inasmuch as the deposition itself cannot 

 be observed. In eggs which were taken from tlie brancliiaj of 

 the Unio or Anodonta^ and apparently had undergone no 

 change after deposition, I usually detected the first traces of 

 the blastoderm in from two to three days. It is formed insu- 

 larly, as may be easily proved by opening an &gg carefully in 

 a solution of 1 per cent, of bichromate of potash. It is im- 

 possible to ascertain the process of formation by the direct 

 observation of the uninjured egg, on account of the dark 

 colour of the yelk. 



After the blastoderm has grown round the whole of the yelk, 

 the embryonal envelope which Cla})ar(^de describes as the 

 deutovum separates from it. This is produced in exactly the 

 same manner as the larval membrane of the Crustacea, as 

 observed by Van Beneden and myself in various species of 

 Gammarus^ . Claparbde was at first inclined to regard f this 

 envelope as the homologue of the structure which in insects 

 has received the unfortunate name of the " amnion ;" but he 

 soon gave up this compaj.*ison. I, on the other hand, regarded 

 the membrane in question in the Mites as homologous with the 

 larval membrane of the Crustacea, and the latter as homo- 

 logous with the " insect-amnion," for which I have elsewhere 

 proposed the better name of " protodcrm." 



Shortly after the formation of the embryonal envelope, we 

 see, between it and the blastoderm, the first amoeboid cells 

 [Jucmamaibai of Claparfede). In the memoir above cited I 

 remarked that these cells " arc blood-corpuscles of quite ab- 

 normal derivation." In using this expression I had the cir- 

 cumstance in ray mind that they are formed from separated 

 blastodermic cells, which, at the time of their production, are 

 the sole cellular structures that we find in tlie egg. I did not 

 then feel it necessary to say any thing more upon this point, 

 as the publication of my original memoir was to be expected. 

 I thought at first that the blood-corpuscles were all developed 

 from separated blastodermic cells, and only afterwards, per- 

 haps after the formation of the buccal orifice, passed through 

 this into the embryo. As, however, I never saw any such 

 migration of the cells, even after observing them for hours, I 

 have given up this view, and now think that there is a further 

 formative focus for them in the interior of the embryo. 



My present opinion as to the ha3mamoeba! is, that they really 

 agree perfectly in form and behaviour with blood-corpuscles, 



* E, van Beneden and E. Bessels, " Rt^sum^ d'un M^moire sur le Mode 

 de Formation dii Blastoderme dans quelqucs groupes de Crustacds," liuU. 

 Acad. Roy. Belg. 2<" s^r. xxv. p. 443. 



t Loc. cit. p. 97. 



