OVUM. 



41 



the minute embryoes, scarcely larger than the 

 blood-discs of the frog, penetrate into the 

 tissues or organs in which they afterwards be- 

 come encysted ; thus affording the most direct 

 proof from observation of the manner in which 

 the young of these parasites become established 

 in the internal parts of animals. The head, 

 with the circle of booklets and the four 

 suckers, is then formed at the anterior part 

 of the embryo, constituting now the scolex of 

 Van Beneden ; and this author proposes to 

 give the name of proscolex to the previous 

 embryonic stage. In connection with the for- 

 mation of the head of the encysted animal, it 

 may also be noticed that Stein has observed 

 the three pairs of embryo booklets remaining 

 for a time irregularly scattered over the en- 

 larged vesicular part of the body.* When 

 the. encysted animal (Cysticercus) has been 

 introduced into the stomach and intestine of 

 a suitable animal, generally a carnivorous one, 

 it resists the digestive action of these organs, 

 but speedily loses its caudal vesicle, and 

 gradually acquires the new joints which are 

 formed from the head. It is thus brought to 

 the condition of the strobila, if we choose to 

 liken the jointed state of the tape-worm to the 

 multiple or divided polype stock from which 

 medusae are thrown off; and lastly, as the 

 sexual organs become fully developed in each 

 of these joints, beginning in the posterior ones, 

 which are first formed, these are detached one 

 by one, and constitute in the separate condi- 

 tion the bodies named proglottis by Dujardin 

 and Van Beneden. The latter author regards 

 these as alone the perfect animals of the 

 Cestoid, the jointed strobila being only a pre- 

 paring stock; and he adheres to the view, 

 previously referred to in this article, that each 

 proglottis approaches in some degree to the 

 organisation of a Trematode animal. Ac- 

 cording to the views of Van Beneden, there- 

 fore, the different stages of a Cestoid worm 

 are the following: 1st, the ovum; 2nd, the 

 first embryo or proscolex; 3rd, the cysti- 

 cercus, or encysted vesicular body, or scolex ; 

 4th, the jointed stock, tape-worm, or strobila; 

 and 5th, the separate sexual individuals, or 

 proglottidea. 



Among the Trematode animals the pro- 

 duction of a succession of new progenies 

 by a process of internal non- sexual ge 

 neration has received full confirmation. Van 

 Beneden has added the important fact, how- 

 ever, that some of the Trematoda are not 

 subject to any process of alternate genera- 

 tion or metagenesis. He has traced the 

 whole process of direct development of the 

 animal from the ovum of Udonella caligorum, 

 a viviparous Trematode with large ova. The 

 alternating generation belongs, according to 

 him, to the Oviparous Trematoda, in which 

 the ova are of small size. 



In reference to the Compound Medusoid 

 Animals, Siphonophora and Physsophorida, 



* In observations made on encysted non-sexual 

 round worms, and the encysted condition of a ces- 

 toid inhabiting the Tenebrio molitor, and its larva, 

 in Zeitsch. fiir Wissen. Zool., vol. iv. p. 206. 



it appears from the researches of Leuckart, 

 Kolliker, Gegenbaur and .others *, that the 

 several joints of the connected chains of 

 these animals, as previously conjectured by 

 Milne, Edwards, and others, may fairly be 

 regarded as distinct, though imperfect in- 

 dividuals, some of which are destined for the 

 sexual production of ova, while the free or 

 floating polypine stock remains destitute of 

 sexual organs. The free polypine stock , is 

 first developed from the fecundated ovum, 

 and acquires its one or two swimming vesicles : 

 from it there is afterwards formed the long 

 chain of smaller bodies connected together by 

 an extension of the digestive cavity and ex- 

 ternal substance of the animal. Some of 

 these form swimming bells or vesicles with 

 prehensile and stinging filaments ; others are 

 digesting cavities or stomachs, and others, 

 most frequently those last formed in the 

 chain, contain one or other of the sexual 

 products, either male and female among the 

 individuals of the same stock, or distinct sexes 

 on separate stocks. But there is sometimes 

 a combination of the motor and nutritive 

 with the reproductive organs in the same in- 

 dividuals of the chain, which warrants fully 

 their being regarded as something more than 

 the mere repetition of organs belonging to 

 one animal. They are, in fact, the same as 

 the sexual individuals of a compound po- 

 lype ; and in some rare instances indeed 

 they have been observed to become detached 

 and to swim about in the separate condition. 

 In connection with the alternating gene- 

 ration of the Salpae, in the discovery of 

 which the whole series of facts now under 

 review may be said to have originated, some 

 doubts were in the previous part of the 

 article stated to prevail as to the rela- 

 tion of the sexes in the animals of the 

 aggregated chain. These doubts have now 

 been in a great measure removed, and the 

 knowledge of the whole phenomena of the 

 reproductive processes in these remarkable 

 animals greatly extended by the researches 

 of C. Vogt f and of H. Miiller.J From these 

 researches it appears that while, as was pre- 

 viously known, the Solitary Salpa3 are en- 

 tirely non-sexual, the animals united together 

 in the aggregated chain, formed by successive 

 gemmations from one spot of each solitary 

 individual, are hermaphrodite, or possess both 

 male and female sexual products. The ova, 

 however, according to Vogt are developed 

 at a much earlier period than the spermatic 

 substance : indeed, they are advanced to the 

 condition in which they are ready for fecun- 

 dation, while their producers are still attached 

 as a chain to their solitary gemmiparous parent. 

 The spermigerous organs only advance to 

 perfection after separation ; and the process 

 of fecundation must therefore be effected in 

 the animals of the attached chain by the 



* Zeitsch. fur Wissen. Zool., vol. iv. p. 304., and 

 vol. v. p. 285. &c. ; and Kolliker's Memoir on the 

 Siphonophora of Messina. Leipzig, 1853. 



t Bilder aus dem Thierieben, 1852, p. 59. 



I Zeitsch. fur Wissen. Zool., 1853, p. 329. 



