ROTIFER A, THEIR STRUCTURE AND REPRODUCTION. 431 



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

 hours more the shell hursts, and the young animal comes forth. 

 In the Rotifer and several other genera, the development of the 

 emhryo takes place whilst the egg is yet retained within the 

 body of the parent (Fig. 201, &), and the young are extruded 

 alive ; whilst in some other instances, the eggs, after their extru- 

 sion, remain attached to the posterior extremity of the body (Fig. 

 200), 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 permanent. The transparency of the egg-membrane, and also 

 of the tissues, of the parent Rotifer, allows the process of deve- 

 lopment 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 ofHydatina (in which genus three 

 or four eggs are deposited at once, and their development com- 

 pleted 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. 

 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 sea- 

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

 eggs" are inferred by Mr. Huxley, from the history of their de- 

 velopment, to be really gemmce produced by a non-sexual opera- 

 tion ; while the bodies commonly called ova, he considers to be 

 true generative products. Dr. Cohn has recently informed the 

 Author, however, that he has ascertained by direct experiment 

 upon those species in, which the sexes are distinct, that the bodies 

 commonly termed ova (Figs. 200, 201), are really internal gemmce, 

 since they are reproduced, through many successions, without 

 any sexual process, just like the external gemmae of Hydra 

 ( 301), or the internal gemmse of Entomostraca and Aphides 

 (Chap. XVI). And this view appears to himself to be more 

 accordant with general physiological analogy than that of Mr. 

 Huxley ; since, in the other instances referred to, as in the Roti- 

 fera, the multiplication by gemmation goes on rapidly whilst 

 food and warmth are abundantly supplied ; but gives place to the 

 true generative process, when the nutritive activity is lowered by 

 their withdrawal. 



280. Certain Rotifera, among them the common Wheel- Ani- 



