TEE OVUM. 



25 



a shell or membrane. In some cases (e.g. Polystomum iutegerrimum) 

 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 com- 

 pletely break up before leaving the yolk-gland. 



There is thus a complete series of gradations between the invest- 

 ment of the ovum by a number of distinct cells, and its investment 

 by a layer of fluid containing 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 Amphistomum subclavatum it presents 

 at one extremity a thickened area, which is pierced by a narrow rnicropyle. 

 In other cases one extremity of the egg-shell is produced into a long pi-ocess, 

 and sometimes 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 Trematodes. 



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 TurbellariansandNemertines, 

 there are greater variations in the arrangement 

 of the female generative glands, than in the 

 preceding types. In most of the Rhabdocoela 

 and fresh-water Dendroccela these organs re- 

 semble in their fundamental characters those 

 of the Trematodes and Cestodes. There are 

 present a paired or single ovary and a paired 

 yolk-gland. The general arrangement of the 

 organs is shewn in fig. 9. 



The blind end of the ovaries is usually 

 (Ed. van Beneden, etc.) stated to be formed 

 of a polynuclear protoplasmic basis, but Hallez 

 (No. 10) has recently insisted that, even at 

 the extreme end of the ovary, the germinal 

 cells are quite distinct, and not confounded 

 together. 



With one or two exceptions the yolk- 

 cells secreted by the vitellarium retain their 

 vitality till they are swallowed by the embryo, 

 after the development of its mouth. The few 



FIG. 9. GENERATIVE 

 SYSTEM OF VORTEX VIRIDIS. 

 (From Gegeubaur, after 

 Mas Sclmltze.) 



t. Testis. v.d. Vasa 

 deferentia. v.s. Seminal 

 vesicle, p. Penis. u. Ute- 

 rus, o. Ovary, v. Vagina. 

 g.i\ Yolk-glands, r.s. Ee- 

 ceptaculum seminis. 



