52 



LECTURE III. 



be no doubt about the functions of the conspicuous ovarium in the 

 latter, for the structure of the ovum can be discerned through its 

 transparent walls {Jig. 22. //): in the Rotifer vulgaris, the young 

 may be seen to escape from the eggs in the uterus, and leave the 

 empty shells behind them : they issue from the parent after intervals 

 of from five minutes to an hour. The unimpregnated eggs are oval, 

 with a firm colourless coat, containing a minutely granular, usually 

 colourless, yolk, with a conspicuous germinal vesicle. In Philodina 

 roseola and Brachionus ruhens the yolk is of a reddish colour. In 

 the tubicular Rotifera the eggs are generally deposited within the 

 tube. In Triarthra and Polyarthra they remain attached to the 

 cloacal opening. In some species two kinds of eggs are laid, one 

 having two shells, and a later period of hatching, carrying the latent 

 life of the germ through the winter, after the death of the parent. 



In the Hydatina senta Ehrenberg has carefully traced the forma- 

 tion of the ova. They are first manifested as clear spots or vesicles 

 filled apparently with albumen. In two or three hours a dark speck 

 is seen in the middle of the clear vesicle, which he compares with the 

 yolk. In five or six hours the yolk fills the clear space and pushes it 

 to one side, and in this state the ova are fecundated and excluded 

 from the cloaca. 



The change in the position of the clear spot is important, from its 

 interesting analogy with the change in the position of the germinal 

 vesicle in relation to the yolk of the rabbit's ovum, and with the 

 altered position of the entire ovum in relation to the ovisac, pre- 

 paratory to impregnation ; both being, to use Ehrenberg's expression, 

 *' pushed to one side ;" to that side, viz., which approximates the 

 important vesicle whence all subsequent development radiates, to 

 the surface which admits the fertilising principle. The result of 

 this admission is seen in the commencement of the series of successive 

 divisions of the yolk, the whole of which is acted upon by the pre- 

 viously dividing central hyaline nucleus, and is so converted into a 

 'germ-mass.' This process of 'total cleavage' of the impregnated 

 egg-material has been described by KoUiker * in 3Iegalotrocha, and 

 by Leydig in Lacinularia^, where the progression is 1, 2, 3, 4, 5, 6, 

 7, 8, &c., instead of 2, 4, 8, 16, 32, 64, as in the usual formation of 

 the germ-mass. The further development of the Rotifera has been 

 very well followed by Ehrenberg. He states, that in the ovum 

 of the Hydatina, three hours after its exclusion, the clear spot (ger- 

 minal vesicle) has disappeared, and the egg is occupied by the yolk, 

 which is granular at one end and clear at the other. A dark spot 



XXXYIII. t XXXY* p. 473. tf. xvii. f. 4. 



