18 OVUM. 



for that of gemmation which takes place 

 throughout the whole of the summer.* 



The ova of Hydra are simple vesicular cap- 

 sules of a brownish colour formed in the sub- 

 stance of the wall of the animal's body, and 

 separated from it previous to the development 

 of the young ; while the spermatic filaments 

 are formed in smaller conical capsules placed 

 nearer to the base of the tentacula either in the 

 same or in different individuals.-}- The for- 

 mation of the young Polype has been observed 

 by Laurent j to take place directly from the 

 internal substance of the ovum, in which, how- 

 ever, he has not traced in a sufficiently complete 

 manner the individual steps of the changes of 

 development (see fig. 1 1 . c.). The origin of the 

 ovum in this animal is shown to be quite dif- 

 ferent from that of a bud : the former having 

 the shape of a distinct vesicle from an early 

 period, the latter not being perceptibly more 

 than an extension of some part of the sub- 

 stance of the wall of the body, and precisely 

 of the same colour and structure (see fg. 

 11. D,D'.). 



The Hydra, therefore, while propagating 

 very frequently by gemmation, is capable of 

 reproduction also by fecundated ova, which 

 are directly developed into the parental form. 

 But many of the true Compound Polypes pre- 

 sent examples, in their multiplication by gem- 

 mation, of the production of intermediate 

 forms of animals between the ova and the 

 perfect sexual individual, a mode of repro- 

 duction, therefore, which may be referred to 

 Steenstrup's general law of Alternate Gene- 

 rations. 



Thus, to begin with the simplest form of 

 these animals bearing the nearest resemblance 

 to the Hydra, in the Coryne and Syncoryne, 

 at certain seasons of the year, multiplication 

 takes place from the stem or root by gemma- 

 tion, the buds being developed in the form of 

 attached Polypes ; but at other times there 

 are developed from the buds, without the con- 

 currence of sexual organs, a set of delicate 

 Medusa-like animals, similar to the Oceania, or 

 those of the naked-eyed kind : these soon be- 

 come detached, swim about freely in the water, 

 acquire some of them male and others female 

 sexual organs, and produce fecundated ova. 



* This effect of the cold season in changing the 

 mode of production from gemmation to oviparous 

 formation, thus checking growth, but providing for 

 the preservation of the species through the winter, 

 is, as remarked by Dr. Carpenter, an interesting ana- 

 logy with the change that is known to occur in the 

 mode of production of the Aphis insect ; see Prin- 

 ciples of Physiology. 



-j- The co-existence of ovigerous and spermigerous 

 capsules on the body of the Hydra has been observed 

 by many, as, first by B. de Jussieu, in 1743 ; (Ab- 

 hand. der Swed. Acad. 1746, vol. viii. p. 211): by 

 Trembley, in 1744 (Mem. sur les Potypes d'Eau 

 douce) ; by Rb'sel (Insecten-Belustigung) j Pallas, 

 in 1776 (Karakteristik der Thier-pflanzen, p. 53) ; 

 and more recently by Ehrenberg, in 1836 and 1838 

 (Verhand. der Naturforsch. FreundeJ in Berlin, 1838, 

 p. 14) ; V.Siebold (Lehrbuch derVergleich. Anat.) ; 

 and by myself (Edin. New Phil. Journ. 1847). 



J Nouv. Rech. sur les Hydros d'Eau douce, 1814, 

 Voyage de la Bonite. 



These ova give rise, by their development, to 

 a ciliated moving embryo : this soon becomes 

 fixed to a spot, and is gradually converted into 

 a Polype, similar to that from which the 

 Medusa-like animals were formed.* 



Fig. 12. 



Syncoryne, developing a 3Iedusoid progeny. Oceania 

 (From Desor.') 



A, natural size. 



B, a portion enlarged, showing the budding of 

 Medusoids in different stages. 



c, one of the Medusoids, naturally detached. 

 D, another, farther advanced ; o t, ovary, or tes- 

 ticle, placed on the alimentary canal ; o', ova. 



R. Wagner appears to have been the first 

 to observe Medusoid bodies produced from 

 the Polype animals, as in Coryne aculeata, in 

 1833f , but the more full observation of the 

 remarkable phenomenon of their formation is 

 due to the researches of Sars, Lowen, Steen- 

 strup, and VanBeneden, who have ascertained 

 the relations of the Polype larva and Medusoid 

 progeny, and the production of ova from the 

 latter. DujardinJ has also carefully traced the 

 production of the free Medusoid bodies from a 

 Syncoryne, which he has called Stauridia, and 

 has farther ascertained the sexual condition of 

 these Medusoids, observed the formation of 

 their ova, and the subsequent development of 

 these ova into Polypes. 



* See fig. of Syncoryna Sarsii, from Sars, Fauna 

 Litt. Norveg. 1846; and Steenstrup's figures of 

 Coryne fritillaria, tab. 1. figs. 41. 43, and Desor, in 

 Ann. des Sc. Nat. 1840, pi. 2. figs. 13. to 16. 



f Isis for 1833, p. 256. Also in Coryne vulgaris, 

 in Icones Zootom. Tab. xxi. 1841. 



I Annal. des Sc. Nat. 1845. 



