GENERAL EMBRYOLOGY. 



according as they are or are not furnished by the queen during 

 oviposition with a spermatozoon. Parthenogenesis is, therefore, 

 not an asexual reproduction which was antecedent to sexual repro- 

 duction, but rather a reproduction which must have been derived 

 from the sexual ; it is a sexual reproduction in which a degeneration 

 of fertilization has taken place. Such facts show that, for the 

 essential point of sexual reproduction, fertilization (the entrance 

 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 sexual cells from the 

 asexual reproductive bodies, the parts arising by division and budding, is 

 shown by their relations to the vital processes of animals. The cells of a 

 bud have had a share in the vital processes of the animal before the begin- 

 ning of reproduction ; they were functional or ' somatic ' cells. In the 

 fresh- water polyp (fig. 90), when a bud arises, the cellular material em- 

 ployed is that which was previously related to the mother animal in 

 exactly the same manner 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, remaining in a resting condition, 

 and conserving their vital energies. Therefore there are also lacking in 

 sexual reproduction 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 example all fishes, continue to grow after the begin- 

 ning of sexual maturity, until they are double or many times their size at 

 that time. Sexual reproduction is not even a special form of growth, but 

 a complete renewal of the organism, a rejuvenescence of it. This explains 

 the important fact that asexual reproduction is most common in the lower 

 animals (ccelenterates, worms), but is lacking from vertebrates, molluscs, 

 and arthropods. The higher the organization of the animal the more 

 the vital energies of its cells must be employed to meet the increasing 

 demands upon their functional capacity, and so the more necessary is 

 sexual reproduction. 



c. Combined Modes of Reproduction. 



Occurrence in the Same Species. Very often two modes of 

 reproduction occur in one and the same species of animal side by 

 side. Many corals and worms have the power of multiplying by 

 division or budding, and also of forming eggs and spermatozoa; 

 again, others have no asexual reproduction, but their eggs develop 

 according to circumstances, either parthenogenetically or after 

 fertilization. The appearance of two kinds of reproduction is very 

 often governed by the fact that individuals with different modes of 



