470 REPRODUCTION OF ROTIFERA. 



maturation is accomplished, renders the multiplication of the race 

 very rapid. The Egg of the Hydatina is extruded from the cloaca 

 within a few hours after the first rudiment of it is visible ; and 

 within twelve hours more the shell bursts, and the young animal 

 comes forth. In the Rotifer and several other genera, the develop- 

 ment of the Embryo takes-place whilst the egg is yet retained 

 within the body of the parent (Fig. 244, &), and the young are 

 extruded alive ; whilst in some other instances the eggs, after 

 their extrusion, remain attached to the posterior extremity of the 

 body (Fig. 243), until the young are set free. In general it would 

 seem that whether the rupture of the egg-membrane takes-place 

 before or after the egg has left the body, the germinal mass within 

 it is developed at once into the form of the young animal, which 

 resembles that of its parent ; no preliminary metamorphosis being 

 gone through, nor any parts developed which are not to be perma- 

 nent. The transparency of the egg-membrane, and also of the 

 tissues, of the parent Rotifer, allows the process of development 

 to be watched, even when the egg is retained within the body ; 

 and it is curious to observe, at a very early period, not merely the 

 red eye-spot of the embryo, but also a distinct ciliary movement. 

 The multiplication of Hydatina (in which genus three or four eggs 

 are deposited at once, and their development completed out of the 

 body) takes place so rapidly, that, according to the estimate of 

 Prof. Ehrenberg, nearly seventeen millions may be produced within 

 twenty-four days from a~ single individual. 



361. Even in those species which usually hatch their eggs within 

 their bodies, a different set of Ova is occasionally developed, 

 which are furnished with a thick glutinous investment ; these, 

 which are extruded entire, and are laid one upon another, so as at 

 last to form masses of considerable size in proportion to the bulk 

 of the animals, seem not to be destined to come so early to 

 maturity, but very probably remain dormant during the whole 

 winter season, so as to produce a new brood in the spring. These 

 ' winter-eggs ' are inferred by Mr. Huxley, from the history of 

 their development, to be really gemmoz produced by a non sexual 

 operation ; while the bodies ordinarily known as ova, he considers 

 to be true Generative products. Dr. Cohn, however, states that 

 he has ascertained, by direct experiment upon those species in 

 which the sexes are distinct^ that the bodies commonly termed 'ova' 

 (Figs. 243, 244), are really internal gemmce, since they are repro- 

 duced, through many successions, without any sexual process, just 

 like the external gemma? of Hydra (§ 411), or the internal gemma? 

 of Entomostraca (§ 502) and Aphides (§ 533) ; whilst the ' winter- 

 eggs ' are only produced as the result of a true Generative act. * 

 And this view appears to the Author more accoidant with general 



* See his important Memoir, 'Ueber dieFortpflanzungderRaderthiere, 

 in "Siebold and Kolliker's Zeitschrift," 1855. 



