DEVELOPMENT OF EMBRYO. 473 



(1235.) The spermatozoa exhibit considerable vivacity in their move- 

 ments, have a disk-like body and a caudal filament, and are propor- 

 tionately of large size. Around them may be seen multitudes of free 

 cellules without caudal appendages, which are apparently young sper- 

 matozoa. In some individuals the spermatozoa are so numerous that 

 the intestinal canal appears completely enveloped by them, and the 

 whole periintestinal cavity seems alive with their movements. 



(1236.) In the mature ovary may be seen ova in different states 

 of development, in each of which the vesicles of Wagner and Purkinje 

 are distinctly visible. In ova approaching their complete maturity, an 

 external vitelline membrane, or chorion, and a vitellus are perceptible, 

 but the two vesicles above-mentioned have disappeared. 



(1237.) When arrived at the proper term, the ova break from their 

 envelope, or ovisac, and escape into the general cavity of the body, where 

 they move freely about, surrounded on all sides by spermatozoa. At 

 length the eggs accumulate in the interior of the parent, near the base 

 of the tentacula ; and their expulsion is ultimately accomplished in the 

 following manner, through a special orifice in the immediate vicinity of 

 the anus : When an ovum is thus about to escape, its external mem- 

 brane is first seen to protrude partially through the aperture, constituting 

 a sort of hernia ; the vitellus then gradually flows from the still enclosed 

 portion of the egg into that which is external ; and when the vitellus has 

 thus entirely passed out, the egg is found separated from the parent 

 animal, and falls into the surrounding water. These eggs are entirely 

 destitute of external cilia, and are carried off by any casual current to 

 attach themselves where chance may bring them ; they are also re- 

 markable for the irregularity of their shape, their form seeming to 

 depend upon the pressure they have been subjected to in the interior of 

 their parent. 



(1238.) In PediceUina, Professor Van Beneden has witnessed the 

 escape of upwards of twenty eggs from a single individual. They are 

 of a pyriform shape, and are enclosed in a pellucid membrane, by the 

 intervention of which they adhere to each other, so that, in the interior 

 of the body of the parent Polyzoon, they have a racemose appearance, 

 and when extruded spontaneously are generally united together in 

 pairs. Between the vitellus and the envelope of the egg there is always 

 a small quantity of a transparent whitish fluid, which doubtless repre- 

 sents the albumen, while the pellucid external membrane itself is the 

 chorion. 



(1239.) The vitellus breaks up into granules, at first of large size, and 

 afterwards, by subdivision, of smaller and smaller dimensions, giving a 

 tuberculated appearance, like that of a raspberry, to the mass. This 

 division seems to be accomplished exactly as in the ova of the higher 

 animals, the yelk first separating into two, then into four, after which 

 its breaking up proceeds- rapidly. 



