OVUM. 



[119] 



phenomena of fecundation been minutely ob- 

 served. 



The formation of these ova may be traced 

 with facility in the substance of the ovary, in 

 consequence of the transparency of the ani- 

 mals. The nucleated germinal vesicle seems 

 first to make its appearance ; the granular 

 }olk substance follows; and the vitelline 

 membrane is last formed.* 



The Rotifera present another example of 

 the formation in the autumn season or before 

 winter, of that variety of the reproductive 

 body which has been called winter egg, and 

 which has already been noticed under the 

 Entomostraca. These bodies were observed 

 by Ehrenberg in Hydatina and Brachionus, 

 by Dalrymple in Notommata, and by Huxley 

 and Leydig in Lacinularia. They are twice 

 the size of the ordinary ova, are formed in 

 very small numbers, probably only two, as is 

 most common in Daphnia, and contain no 

 apparent germinal vesicle. Mr. Huxleyf- ap- 

 pears to have pointed out very clearly the dis- 

 tinction between true or ordinary ova and these 

 reproductive bodies. He says, at p. 16 of his 

 paper, " The true ova are single cells which 

 have undergone a special development, the 

 ephippial ova are aggregations of cells, in fact 

 larger or smaller portions, sometimes the 

 whole of the ovary, which become enveloped 

 in a shell and simulate true ova." Mr. Huxley 



Fig. 84*. 



Formation of Ephippial ovum in Lacinularia 

 Socialis. (From Huxley.) 



A. represents a portion of the ovary massed 

 together and undergoing a change of structure pre- 

 paratory to its conversion into the ephippial ovum. 



B. the ovum now complete, the external invest- 

 ment distinct. 



c. the same having now its contents divided into 

 two portions. The ephippial ova differ from the 

 ordinary ones in their mode of formation, and in 

 having'three investments. 



has traced minutely the process of conversion 

 of the substance of the ovary into such an 

 ephippial ovum, or rather the protecting 

 covering of the two ova which are contained 

 in the ephippium ; and his observations seem 

 to show a manifest difference between these 

 and the ordinary ova. The same follows 

 also from Mr. Dalrymple's researches on No- 

 tommata. The correspondence of the num- 



* Leuckart, loc. cit. 

 t Loc. cit. 



ber and general structure of these ova in 

 Daphnia and the Rotifera is also deserving of 

 notice. These winter ova, besides being much 

 larger than the ordinary ones, differ from them 

 also in structure, having three investing 

 membranes; and they appear designed, like 

 those of the same kind in other animals, to 

 resist the cold of winter and other hurtful 

 influences. 



It would appear that these ephipphial ova, 

 like those of Daphnia, do not require fecunda- 

 tion. Leydig, though distinguishing the two 

 kinds of ova, regards the hybernating ova as 

 only modifications of the ordinary ones ; 

 while Huxley considers them rather as pecu- 

 liar buds like those of Aphis or Gyrodactylus.* 



Turbellaria. Under this class three orders 

 of the animals allied to the Planaria may be 

 brought, according to the researches of Qua- 

 trefages and others, viz., the two kinds of 

 Planaria with simple and ramified alimen- 

 tary canal, or Rhabdoccela and Dendrocoela, 

 and the Nemertides or Mioccela. The first 

 two orders are hermaphrodite ; in the third 

 the sexes are distinct. The ovology of this 

 class is known principally from the interesting 

 and beautiful researches of Quatrefages f; but 

 the history of the structure and formation of 

 the ova is still far from being complete. 



The ova of the Planariae are of various 

 magnitudes, and present some differences in 

 their structure. For the most part they con- 

 tain only the finely granular yolk, but with oc- 

 casionally some oil globules interspersed. It 

 is only in the earliest stages that the germinal 

 vesicle is perceived with ease, in consequence, 

 probably, of the opacity of the yolk-substance, 

 and the dark colour of the external envelopes. 



In most of the genera the germinal vesicles 

 and the yolks are formed in separate organs, 

 as in the trematode animals, to which the 

 Planarias are nearly allied, but in some, as 

 Macrpstomum, these two organs come to be 

 combined in one. At first the yolk-mass, in 

 descending and meeting with the germinal 

 vesicles, unites a number of them into a con- 

 nected chain ; but somewhat later the ova 

 are separated into distinct spheres, and a 

 vitelline membrane is formed to enclose each 

 of them. 



Just as occurs in the body of the adult 

 Planariae, there is also in the ova a remarkable 

 tendency to subdivision by fission. Thus, in 

 the commencement of the development of 

 the ovum, it is liable to become divided into 

 distinct masses, so as to give rise to the de- 

 velopment of a number of embryoes from one 

 ovum. Such, at least, is the view entertained 

 by some ; but there may be doubts as to 

 whether the ovum so divided is really simple, 



* See on this subject also, Burnett's translation 

 of Yon Siebold's Comparative Anatomy, p. 150. 



f Me'm. sur quelques Planarie'es Marines, in 

 Annal. des Scien. Nat. 1845, torn. iv. p. 169 ; and 

 Me'm. sur la Famille des Nemertiens (Nemertea), 

 id. lib. 1846, torn, vi., p. 269. The Rhabdocoala 

 are known chiefly by the researches of Schmidt, 

 Die Rhabdocoelen Strudelwiirmer des siissen Was- 

 sers, Jena, 1848; and of Schultz, Beitrage zur 

 Geschichte der Turbellarien, 1851. 

 M] 



