GENERAL EMBRYOLOGY. I'JI 



which are not actual eggs. This view is absolutely untena- 

 ble in view of the proof that the "pseud-ova" arise just 

 like ordinary eggs and develop like them, since they cleave 

 and form germ-layers. The equivalence of partheno- 

 genetic eggs to those which are fertilized is best shown in 

 the case of the bee, where similar cells give rise to a female 

 or a male insect according as they are or are not furnished 

 by the queen during oviposition with a spermatozoon. 

 Parthenogenesis is, therefore, not an asexual reproduction 

 which precedes sexual reproduction, but rather a reproduc- 

 tion which must have been derived from the sexual; it is 

 a sexual reproduction in ivJiich a degeneration of fertiliza- 

 tion has taken place. In consideration of this condition, we 

 must accustom ourselves to the conception that, for the 

 essential point of sexual reproduction, fertilization (the en- 

 trance of the spermatozoon) forms indeed an extremely 

 important, but a by no means indispensable, characteristic. 

 To all cases comprised under amphigony this definition 

 alone applies: "sexual reproduction is a reproduction by 

 means of sexual cells." 



Sexual and Somatic Cells. The distinction of sex- 

 ual cells from the asexual reproductive bodies, the parts 

 arising by division and budding, is shown by their rela- 

 tions to the vital processes of animals before the beginning 

 of reproduction. The cells of a bud have had a share in 

 the vital processes of the animal ; they were functional or 

 "somatic" cells. In a fresh-water polyp, when a bud 

 arises, the cellular material employed is that which was used 

 previously by the mother animal exactly in the same man- 

 ner as the other parts of the body- wall. The sexual cells 

 of an animal, on the contrary, are permanently, or at least 

 for a long time, excluded from the vital processes, remain- 

 ing in a resting condition, and conserving their vital ener- 

 gies. Therefore, there are lacking also in sexual repro- 

 duction the relations to growth which are so remarkable in 

 asexual reproduction. For, although very often sexual 

 reproduction does not begin until the bodily growth is 

 completed, yet it is found repeatedly that animals, as for 



