o/MeRhizopoda. 861 



spot ; secondly, the appearance of the germinal Vesicle ; thirdly, 

 the advent of the macula within the vesicle ; and, lastly, the 

 formation of the enclosing membrane. 



On examining a great number of specimens of Gromia, Milio- 

 lina, Rotalina, and Orbulina, I have repeatedly discovered bodies 

 which correspond in all respects with the "primitive ovum'' 

 defined above. They consist of transparent spheres or ovoids 

 formed of a finely molecular substance, but in which the mole- 

 cules are masked or rendered indistinct by the highly refractive 

 matter in which they are imbedded. No germinal vesicle or 

 spot appears in the living specimens. It may be masked in a 

 similar manner to the molecular structure ; but in a specimen 

 of TruncatuUna (PI. XVIII. fig. 6), wliich has been hardened in 

 spirit, decalcified by dilute nitric acid, and mounted with strong 

 heat in Canada balsam, four of the segments or zooids contain 

 each an ovum which shows a germinal vesicle and spot with the 

 utmost distinctness, while the rest present the usual appearance 

 of granular, low-refracting sarcode*. 



I have not been able to trace the development of the ova of 

 llhizopods. Bodies similar to those I have considered ova in 

 Gromia are found attached to Algse in vessels where that animal 

 abounds. The ova of Gromia are very small ; and young Gro- 

 mias slightly larger than the ova also occur. In Gromia, there- 

 fore, the ova may be at once transformed into young, and 

 directly acquire an envelope. Such is the mode of development 

 in the ova of most of the Hydroidse, which are transformed into 

 planuloid larvae without undergoing fissure. In Orbulina, how- 

 ever, the ovum is of very large size, and consists of a colourless 

 spherule of sarcode enclosed in a membranous test and covered 

 by a thin glairy layer. Here the sarcode presents traces of fis- 

 sure, though these are lost when it is pressed out of its envelope. 

 In both this genus and TruncatuUna it is impossible that the 

 full-sized ova can obtain exit from the animal, except by the 

 destruction of the chambers of solid shell in which they are en- 

 closed. In the case of TruncatuUna, moreover, the ova are at 

 least ten times as large as the primordial segment or zooid of 

 the adult. It is therefore probable that the ova of these genera 

 undergo a "polymorphic" development of many months' dura- 

 tion, similar to that described by Carter as occurring in Amoeba 

 verrucosa-]', and that each ovum becomes transformed into nume- 

 rous Amoeboid zooids, which escape through the openings of the 

 shell and form the primordial segments of future llhizopods. 



* I shall be happy to lend this prepar.itiou to any gentleman who may 

 take an interest in it. 

 t Ann. Nat. Hist. ser. 2. vol. xx. p. 37. 



Ann. ^ Mag. N. Hist. Ser. 3. Vol. vii. 24 



