ELEMENTARY VITAL PHENOMENA 



199 



and others, who have thrown light upon the minute details of the 

 phenomena of fertilisation. 



In the human being and the higher animals the process of fer- 

 tilisation cannot be observed, because it is concealed in the inte- 

 rior of the female body, and it is not possible to keep the egg- cells 

 alive outside of the body and there fertilise them with sperm. This 

 latter method, however, succeeds with certain lower animals, and 

 hence in eggs that are particularly large and transparent, such as 

 those of the sea-urchin and the round-worm of the horse, the 

 whole course of this interesting process has been carefully studied. 



As has already been seen, the male and the female germ-cells 

 are differentiated very differently. While the ova usually are 

 large, spherical or amoeboid cells consisting of a vesicular nucleus 

 and much protoplasm, the latter containing the building-materials 

 for the future development (Fig. 80), the spermatozoa are ex- 



FIG. 80. Ova. /. Spherical ovum of a sea-urchin. (After Hertwig.) II. Amreboid ovum of a 

 calcareous sponge. (After Haeckel.) 



tremely tiny in comparison with them. The spermatozoa consist 

 chiefly of nuclear substance, and have only a thin protoplasmic 

 covering; in most cases the latter is extended into a motile 

 flagellum, the tail, which is distinguished from the rest of the 

 body, the head, and serves for the movement of the spermato- 

 zoon in seeking the ovum. The finer structure of the sperm-cell, 

 as the detailed investigations of Ballowitz ('90) have recently 

 shown, is very complicated, and very various differentiations occur 

 among different animals. The accompanying illustrations present 

 some examples of this (Fig. 81). But both the spermatozoa and 

 the ova are always complete cells, and contain both the essential 

 cell-constituents, protoplasm and nucleus a fact upon which 

 special emphasis should be laid. 



Before fertilisation takes place, in some cases also during the 

 beginning of fertilisation, there occurs the maturation of the ovum, 

 which consists in the formation, by means of two successive 

 divisions of the nucleus, of two buds, the polar bodies or direction- 



