THE OVUM. 



nucleus, or render it very difficult to see. In the majority of 

 cases the entire cells forming the lining of the vesicles constitute 

 the secretion of the yolk-gland. They invest the ovum, and 

 around them is formed a shell or membrane. In some cases 

 (e.g. Polystomum integerrimum) the yolk-cells retain their 

 cellular character and vitality till the embryo is far developed. 

 In other cases they lose their membrane and nucleus shortly 

 after the formation of the egg-shell, and break up into a fluid, 

 holding in suspension a number of yolk-granules. A partial 

 disorganisation of the yolk-cells can also take place before they 

 surround the ovum ; while in some species of Distomum they 

 completely break up before leaving the yolk-gland. 



There is thus a complete series of gra- 

 dations between the investment of the 

 ovum by a number of distinct cells, and 

 its investment by a layer of fluid con- 

 taining yolk-spherules in suspension. In 

 neither the one case nor the other do the 

 investing structures take any share in the 

 direct formation of the embryo from the 

 ovum. Physiologically speaking they play 

 the same part as the white in the fowl's 

 egg- 



The egg-shell, which is usually formed by a 

 secretion of a special shell-gland opening into the 

 oviduct, exhibits one or two peculiarities in the 

 different species of Trematodes. In Amphisto- 

 mum subclavatum it presents at one extremity a 

 thickened area, which is pierced by a narrow mi- 

 cropyle. In other cases one extremity of the egg- 

 shell is produced into a long process, and some- 

 times even both extremities are armed in this way. 

 Opercula and other types of armature are also 

 found in different forms. 



The mode of development of the ovum in 

 Cestodes is very nearly the same as in Trema- 

 todes. 



The ovum becomes enveloped in the usual secretion of the yolk-gland ; 

 and an egg-shell is always formed by the secretion of a special shell-gland. 



Amongst the Turbellarians and Nemertines, there are greater 

 variations in the arrangement of the female generative glands, 



FIG. 9. GENERATIVE 

 SYSTEM OF VORTEX VIRI- 

 Dis. (From Gegenbaur, 

 after Max Schultze.) 



t. Testis. v.d. Vasa 

 differentia, v.s. Seminal 

 vesicle, p. Penis, u. Ute- 

 rus, o. Ovary, v. Vagina. 

 g.v. Yolk-glands, r.s. Re- 

 ceptaculum seminis. 



