ROTIFERA. 37 



the ova being large and few in number : a few are ovo-viviparous. 

 The fertilising principle is formed in and by two long and slender 

 tubes (^Jig. 15, A, ^), commencing each by a blind extremity at the ante- 

 rior part of the abdominal cavity, and extending with a few slight 

 folds to the neck of a single large spermatic vesicle, which communi- 

 cates with the oviduct in the cloaca. The essential organ of the male 

 apparatus is thus manifested under its most simple form, as a single 

 tuhidus in each testis^ in these small animals. The singleness of the 

 vesicula, which is characterised by the same remarkable irritability as 

 the two contractile vesiculae of the Polygastria^ accords with the ab- 

 rogation of the fissiparous property in the Rotifera. 



The egg-forming organ consists of a simple wide sac, single in 

 Notommata {fig- 15, I), but more commonly divided into two cornua, 

 the body terminating by a short contracted cervix, which communi- 

 cates with the cloaca. There can be no doubt about the proper func- 

 tion of this conspicuous viscus, for the structure of the ovum can be 

 discerned through its transparent walls ; and, 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 in- 

 tervals of from five minutes to an hour. 



In the Hydatina senta Ehrenberg carefully traced the development 

 of the ova and embryons. The ova 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 com- 

 pares with the yolk. In five or six hours the yolk fills the dear 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 im- 

 portant vesicle or cell whence all subsequent development radiates, 

 to the aperture which admits the fertilising principle. 



Ehrenberg states that in the ovum of the Hydatina, three hours 

 after its exclusion, the clear spot (germinal 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 then appeared in the middle of 

 the ovum, which, six hours after exclusion, could be distinguished as 

 the head with the rudimental dental aj^paratus of the embryo. At the 

 eleventh hour the wheel-like ciliated organs began to play, and the 



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