OVUM. 



[129] 



near the anterior extremity of the cell. A 

 number of ova were found in the ovary con- 

 taining the distinct germinal vesicle with 

 macula. He also observed the segmentation 

 of these ova in the usual manner, and the 

 conversion of the segmented mass into a 

 ciliated embryo, within which the new polype 

 is subsequently developed.-f- 



Should these observations prove correct 

 and be applicable to the other instances of 

 similar winter ova among the Bryozoa, they 

 may tend to remove some of the difficulties 

 which exist in regard to the various repro- 

 ductive bodies occurring in these animals ; 

 but farther researches seem still necessary to 

 point out in these and in other polypine ani- 

 mals more fully and minutely the relation be- 

 tween the three kinds of reproductive bodies, 

 viz., true ova, separated gemmules, and at- 

 tached buds. 



AcalephcB. It is remarkable that notwith- 

 standing the very close relation in which these 

 animals stand to the Anthozoid Polypes, the 

 form of their ova is not the same. The Dis- 

 cophora (Medusae) are of distinct sexes : the 

 Ctenophora (Beroes) are hermaphrodite ; the 

 Siphonophora (Diphyidae) are various, or bear, 

 in the manner of compound animal stocks, a 

 variety of zoids, sometimes of one sex alone, 

 at other times of different sexes on the same 

 stem. 



The structure of the ova in Medusas is 

 extremely simple. They are originally formed 

 from minute cytoblasts which soon acquire a 

 single nucleus or macula, and are enclosed in 

 a delicate external membrane. These consti- 



Fii*. 95*. 



Development of the ova of Acalepha. 



These figures give magnified views of the diffe- 

 rent stages of formation of the ova taken from the 

 ovary of a large Rhizostoma. a. The primitive 

 germ. b. The germinal vesicle now present in the 

 primitive ovum. c. d. The same more advanced 

 and enlarged, the macula has appeared in the ger- 

 minal vesicle, and a few yolk granules are deposited 

 in the clear vitelline substance, e. The yolk gra- 

 nules greatly increased in quantity and becoming 

 opaque, a vitelline membrane is now formed, f. 

 The same somewhat more advanced, the yolk gra- 

 nules are now collecting together to form cor- 

 puscles. The macula is assuming the elongated 

 form. 



t Proceedings of British Association for 1855. 

 See also Professor Allman's interesting Report on 

 the Polyzoa to the British Association. See 

 Trans, for 1850, p. 320. 

 Supp. 



tute the germinal vesicles, round which the 

 granular yolk-substance is gradually deposited 

 in increasing quantity. The complete segmen- 

 tation of the yolk has been observed by Von 

 Siebold in Cyanea aurita.* The yolk-sub- 

 stance is often highly coloured, violet or 

 yellow. In the former part of this article I 

 have referred to the manner in which some 

 compound Hydroida are propagated through 

 their medusoid progeny. These medusoid 

 individuals, like the ordinary Medusae, are 

 of separate sex ; and they must therefore 

 be looked upon as the complete stage of 

 the polypine animals from which they have 

 proceeded, whether they have their young 

 developed while the parent remains at- 

 tached to the nursing polype stock, or have 

 assumed the separate and independent mode 

 of life in a more complete state of develop- 

 ment. There are many varieties in the de- 

 gree of perfection to which they attain even 

 while remaining attached to the polype ; but 

 the general principle of formation is the same 

 throughout the whole of the hydroid animals, 

 the remarkable and constant fact with regard 

 to the mode of their reproduction being this, 

 that the immediate product of development 

 from the ovum which has been formed by 

 sexual generation from a Medusa or medusoid 

 animal is invariably an attached Polype, and 

 that the medusa or medusoid is the product 

 of a non-sexual process of gemmation from 

 this polype stem. 



Protozoa. With regard to the Protozoa, or 

 Infusoria and Rhizopoda, it is unnecessary 

 to add anything here to what has been stated 

 in the several articles on these subjects and in 

 a former part of this one, excepting the remark, 

 that continued researches appear to show that 

 as the sexual distinction has not been de- 

 tected, and may probably be absent in these 

 animals, the nucleus of the monocellular 

 forms of these beings may hold the place of 

 the germinal vesicle in them, and that the 

 processes of division and production of in- 

 ternal gemmules takes the place of true ovu- 

 lation. At the same time it must be admitted 

 that it is by no means improbable that the 

 sexual relations may yet be discovered in the 

 lowest monocellular animal bodies, as has re- 

 cently been the case in some of the simpler and 

 monocellular Alga?, and that as our knowledge 

 of the process of reproduction in these beings 

 is still very limited, it may be destined to un- 

 dergo even greater progressive changes than 

 those which it has suffered from the researches 

 of the last few years.f 



Porifera. The bodies which have usually 

 been regarded as the ova of Sponges, and 

 to which a reference was made in the earlier 

 part of this article, are of two kinds, viz. gem- 

 mules or detached ciliated portions of the 



* Beitr. zur Naturgesch. der Wirbellos. Thiere, 

 1839. 



f See the papers of Focke, Cohn, and Stein re- 

 ferred to in the first part of this article, and the 

 more recent work of Stein, "Die Infusionsthiere 

 auf ihre Entwickelungsgeschichte untersucht." 4to. 

 Leipzig, 1854. 



M 



