104 COMPARATIVE ANATOMY. 



iform, with fine hair-like tail, as in Aves and Reptilia v ; 

 staft-shaped expansion, with short filament often coiled upon 

 itself, as in Batrachia generally; globular, as in osseous 

 fishes; vermiform, as in marsipobranchiate, elasmobranchiate 

 fishes, and cephalopodous molluscs; and acuminate-oval 

 with long filament, as in Helix (snail), with short cylindrical 

 or pyriform fibres, as in Unio (mussel). In Nepa the expan- 

 sion is slight, and filament long, cylindrical in Epeira (spi- 

 der), pyramidal, with concave sides in Julus, or of a gro- 

 tesque shape, with central body and radiatory filaments, as in 

 Homarus (lobster), and other decapod crustaceans. In Onis- 

 cus (pillbug), the spermatic filaments are large enough to be 

 seen by the naked eye. The quantity is at all times greatly 

 in excess of the number of eggs actually impregnated. It is 

 estimated that in a drachm of human semen more spermatic 

 filaments are contained than there are people in the world. 



OVUM. 



The ovum (egg) is the product of parental sexual genera- 

 tion from which the young of animals are produced. (Thom- 

 son.) 



The simplest expression of the egg is a sphere with faintly 

 granular contents, as in Stentor. It may present two concen- 

 tric spheres, as in Laomedea; or in the higher animals, as 

 most molluscs, articulates and vertebrates, three concentric 

 spheres, called after their position from within outward, nu- 

 cleolus, germinal spot (nucleus, Wagnerian vesicle), and 

 germinal vesicle (germ cell, vesicle of Purkinje). 



Every egg is composed of an albuminous and oleaginous 

 substance. The first of these is on the side of the ovum 

 with the germinal spot and vesicle, which is hence called the 

 germinal area; the second is upon the opposite side, and is 

 known as the yolk (vitellus). Ordinarily the yolk is formed 

 with germinal vesicle in ovary, but in Holothuria (sea-cucum- 

 ber) and in certain Entozoa (q.v.) distinct appendages (yolk 

 glands) to the oviduct are reserved for this purpose. 



The egg becomes impregnated by the entrance of one or 



