GERM-CELLS : FERTILIZATION OF GERM. 559 



from other cells by the nature of their contents, though they are 

 usually recognisable by the peculiar nuclei they present ; each 

 cell is known as the germinal vesicle (fig. 302, d), whilst its 

 nucleus (e) is designated the germinal spot. The act of fertiliza- 

 tion appears to consist in the contact of one or more spermato- 

 zoids with the exterior of the germinal vesicle ; the sperma- 

 tozoids, ceasing to move, undergo a sort of liquefaction ; and 

 the product of their dissolution, being received by absorp- 

 tion into the interior of the germinal vesicle, mingles with its 

 contents, to form with them the basis of the new structure. 

 When, as usually happens, the germ-cells are developed in a 



Fig. 302. SECTION OF OVAB.IUM OF FOWL: 



a, fibrous substance of the ovary; b, yolk; c, yolk-bag; d, germinal vesicle; e, 

 germinal spot. 



special and distinct organ, this organ, which is termed the 

 Ovary, has very commonly among the lower animals a glan- 

 dular character, the mature ova being discharged by the ovi- 

 duct, just as the products of secretion pass-off through the 

 ducts of their respective glands : but among the Vertebrata 

 the ovary has a much more solid texture, and the germ-cells, 

 developed in the very midst of its fibrous tissue (fig. 302, d), 

 have to find their way to its surface, and to burst forth from it ; 

 being then received into an oviduct, whose trumpet-shaped 

 mouth embraces the ovary, so as to prevent the liberated 

 germs from falling (as they would otherwise do) into the 

 visceral cavity of the body. 



733. "With the "germ-cell" there is always associated in 

 Animals, as in Flowering Plants, a store of nutriment that 

 serves for the early development of the germ ; this consists 

 of a mixture of albuminous and oily matter, known as the 

 yolk (fig. 302, b) ; and it is inclosed in a membranous envelope c, 



