60 



POPLITEAL REGION. 



looks upon as being the testis, his opinion 

 being founded on the fact that when a mature 

 specimen of the animal is placed between two 

 plates of glass, and gently compressed so as 

 to rupture its parietes and cause the escape 

 of the viscera, spermatozoa are discoverable 

 in the one and ova in the other. The sper- 

 matozoa exhibit considerable vivacity in their 

 movements, have a disc-like body and a cau- 

 dal filament, and are proportionately of large 

 size ; around them may be seen multitudes 

 of free cellules without caudal appendages, 

 which are apparently young spermatozoa. 



In some individuals the spermatozoa are so 

 numerous that the intestinal canal appears 

 completely enveloped by them, and the whole 

 peri-intestinal cavity seems alive with their 

 movements. 



In the mature ovary ova are discoverable in 

 different degrees of development, in each of 

 which the vesicles of Wagner and of Purkinje 

 are, according to Professor Van Beneden, dis- 

 tinctly visible. In those ova which approach 

 their complete maturity an external vitelline 

 membrane, or chorion, and a vitellus are per- 

 ceptible, but the two vesicles above men- 

 tioned have disappeared. 



When arrived at the proper term the ova 

 break from their envelope, or ovisac, and fall 

 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 body, near the base of the 

 tentacula, and their escape, as witnessed by Van 

 Beneden in Laguncula repens, is at length ac- 

 complished in the following manner. An egg 

 presents itself at an orifice, situated in the 

 vicinity of the anus, through which its external 

 membrane partially protrudes, constituting a 

 sort of hernia (fig. 61, p). The vitellus gra- 

 dually 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 remarkable for the irre- 

 gularity of their shape, some being completely 

 angular, their form seeming to depend upon 

 the pressure they have been subjected to in 

 the interior of their parent. 



Development by ova. In Pedicellina Pro- 

 fessor Van Beneden has witnessed the escape 

 of upwards of twenty eggs from a single indi- 

 vidual. They are of a pyritbrm figure, and 

 enclosed in a pellucid membrane, by the in- 

 tervention of which they adhere together (fig. 

 66. 1), so that in the interior of the body of 

 the parent Bryozoon they have a racemose 

 appearance, and when extended spontaneously 

 they are generally united together in pairs. 

 Between the vitellus and the envelope of the 

 egg there is dways a small quantity of a trans- 

 parent whitish fluid, which doubtless repre- 

 sents the albumen, while the pellucid external 

 membrane itself is the chorion. 



The vitellus breaks up into granules, at 

 first of large size, and afterwards by sub- 



division 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 yolk first separating 

 into two (fig. 66. 3), then into four (fig. 66. 

 2), after which its breaking up proceeds 

 rapidly (fig. 65. 4). 



Fig. 66. 



A series \illustratmg the development Ly ova of Pe- 

 dicellina. (After Van Beneden.) 



The embryo enclosed within the egg soon 

 assumes a rounded form and speedily appears 

 divided by two indentations near its middle 

 (fig. 66. 5), by which it is separated into an 

 anterior and a posterior moiety, and vibratile 

 cilia become apparent upon the anterior ex- 

 tremity. 



That portion where the cilia have become 

 apparent insensibly enlarges and assumes the 

 shape of a funnel (fig. 66. 6), while the long 

 cilia by which it is fringed begin to keep the 

 particles suspended in the water around them 

 in rapid motion. The margins of the funnel 

 gradually extend themselves (fig. 66. 7), the 

 body exhibits frequent contractions, and at 

 the end of about two hours little tubercles 

 become apparent upon its anterior extremity, 

 which subsequently become developed into 

 the tentacula. Professor Van Beneden thinks 

 that the original cilia disappear when the ten- 

 tacula have become developed and furnished 

 with their proper vibratile apparatus. The 

 formation of the tentacula at once indicates 

 which are the two extremities of the body and 

 the point by which the embryo will subse- 

 quently attach itself. 



The embryo when mature is quite at liberty 

 and strikingly resembles some forms of Infu- 

 soria, but after a little while a pedicle is 

 formed, whereby it proceeds to fix itself to 

 some foreign body, and thus permanently 

 assume the aspect of its race (fig. 66. 8). 

 The pedicle seems to be formed by a cell 

 developed below the stomach, which grows 

 directly outwards, and thus completes the 

 organisation of the young Bryozoon. 



( T. Rymer Jones.) 



POPLITEAL REGION, and POPLI- 

 TEAL ARTERY. The term POPLITEAL 

 REGION is applied to that portion of the 



