256 T. W. Engelmann on the Infusoria. 



verse fission of Didinium, the nucleus becomes much elongated, 

 and then appears striped longitudinally ; but on the completion 

 of fission the longitudinal striae disappear. 



In the group of Acinetina, development by internally produced 

 embryos has been demonstrated by Stein and Claparede in 

 almost all the species known. Engelmann remarks that he has 

 seen the embryos of Acineta opercularice, A. quadripartita, A. 

 Astaci, and A. infusionum. In the last-named species and in 

 A. quadripartita, Engelmann further observed the reverting of 

 the ciliated embryos to Acinetse ; and he remarks that the em- 

 bryos of Acinetse known to him are distinguished from those of 

 ciliated Infusoria (e. g. Stylonychia Mytilus, Paramecium aurelia, 

 Epistylis plicatilis, &c.) by the remarkable circumstance that 

 their entire body is not, as in those of the latter animalcules, 

 devoid of granules, and transparent, but filled with the same 

 nutritive materials as occur in the parent beings. For example, 

 the embryos of Acineta quadripartita and of A. infusionum are 

 so filled with the fat-globules and other nutritive matters derived 

 from the parent animals, that it is difficult to discover their 

 nucleus and contractile sac. Indeed, this single circumstance 

 renders it highly improbable that the entire Acineta-embryo can 

 be derived solely from the substance of the parent nucleus. 

 This last is certainly frequently seen broken up into numerous 

 coarse granules, but only a small portion at most of the fre- 

 quently extremely large and opake embryo can be derived from 

 them; on the contrary, it is much more probable that only 

 their nuclear portion arises from the nucleus of their parent, 

 the rest of their large body being derived directly from the con- 

 tents of the parent Acineta. Stein, in fact, has hei'etofore inti- 

 mated the same. Thus in Acineta infusionum he describes the 

 nucleus as giving off from its middle, at right angles to itself, a 

 process, of a bud-like form, which pi-esently becomes surrounded 

 by a clearer ring of tissue derived from the substance of the 

 parent being. Engelmann observed precisely the same thing 

 take place in a large specimen of Acineta quadripartita. 



The origin of the offspring of Acinetina by sexual reproduc- 

 tion is highly improbable, and has no support from observation. 

 It is probably an act of internal gemmation. It is only in 

 Acineta mystacina, Urnula epistylidis, and Podopkrya fixa that 

 the production of these internal germs has been unnoticed ; and 

 as far as our knowledge at present extends, these three species 

 multiply only by simple fission. But even in other Acinetina the 

 internal production of germs is not an act of embryonic repro- 

 duction, but simply one of fission. 



A second variety of reproduction, however, does occur among 

 the Acinetina, as first noticed by Claparede in Acineta quadripartita 



