2 I 8 ROTIFERA 



at the cloaca (B) has been seen by many observers; but it 

 appears equally certain that in many cases the male bores into 

 the body-wall of the female at any point, and deposits the sper- 

 matozoa in the body-cavity, so that they must pass through the 

 wall of the oviduct to effect fertilisation. Maupas finds that 

 the process of fertilisation is ineffective except upon such newly- 

 hatched females as would otherwise be the parents of small male 

 eggs; that fertilisation is inoperative even for these at a later 

 age when their eggs have begun to mature ; and that it is 

 wholly useless for those that lay ordinary summer eggs. The 

 parent of male or winter eggs would thus be comparable to the 

 queen bee, which if not fertilised produces drones. These sexual 

 relations find a close parallel in the Ostracod and Phyllopod 

 Crustacea, as well as in many plant-lice (Homoptera). 



Development. This has only been fully studied in the 

 summer egg ; in Brachionus by Salensky, 1 in Melicerta by 

 Joliet 2 ; in Eosphora digitata and several other species by Tessin 3 ; 

 in Callidina and Melicerta by Zelinka, 4 the last two observers 

 having utilised modern methods of research. We shall base our 

 account on Zelinka's observations. As in the case of most 

 " parthenogenetic " eggs, the ovarian egg begins by a very uneven 

 division to form two cells : the minute " first polar body " which 

 undergoes no further development ; and the definitive egg, which 

 by its repeated divisions gives rise to the tissues and organs. 



Segmentation is very unequal, and recalls that of Molluscs 

 in several respects. The first division gives rise to a smaller 

 and a larger cell. Both of these divide again, the latter un- 

 equally, so that now there are three smaller cells and one large 

 one ; and after repeated divisions of the small cells and unequal 

 divisions of the larger one, a stage is reached where there are a 

 number of small cells and one large one, which sinks in and is 

 overgrown by the small ones. Just prior to this the large cell 

 undergoes equal divisions; its cells are the " hypoblast " cells 

 (Fig. 114, hyp), and give rise to the gullet, stomach, and intes- 

 tine, with their appendages, and the generative organs; while 

 the smaller cells constitute the " epiblast " (ep), which gives rise to 

 the body-wall and muscles, to the cement glands, nervous system, 

 pharynx and mastax, and probably to the kidneys. 



1 Zeitschr. wiss. Zool. xxii. 1872, p. 455. 2 Arch. Zool. Exp. str. 2, i. 1883, p. 131. 

 3 Zeitschr. wiss. Zool. xliv. 1886, p. 273. * Ibid. liii. 18"92, p. 1. 



