PARTHENOGENESIS 143 



in the case of two cells lying opposite to one another, though not 

 in contact, and though each is enclosed in a firm cell-wall, some 

 stimulus is transmitted from one to the other which calls forth 

 a definite response manifested in the formation of the connecting 

 canals and the conversion of the protoplasmic contents of the 

 cells into gametes. The insufficiency of the principle of chemo- 

 taxis to account for these phenomena appears to be indicated 

 by the fact that cells which have no mates do not form either 

 complete connecting canals or gametes, though apparently 

 exposed equally with their neighbours to the influence of any 

 chemical substances dissolved in the surrounding water. A 

 possible explanation appears to be that the solitary cell cannot 

 attract, or at any rate retain, the attention of a mate to stimulate 

 it to complete the process of conjugation. 1 



The cytotropic attraction of the gametes, as we have already 

 observed, probably depends upon some difference of polarity 

 between the two. That it is mutual is demonstrated by such 

 cases as that of Zygogonium (see p. 97) and by the fact that even 

 when the ovum is too heavily laden with food material to take any 

 active part in the process of conjugation it yet in many cases puts 

 out a definite " cone of attraction " towards the advancing sper- 

 matozoon, as seen in Coccidium (Fig. 39). What the real nature 

 of this primary attraction between the gametes is we do not know ; 

 it may ultimately be explicable in terms of some force already 

 known to us, or it may be one of those cases where it will be 

 convenient to cloak our ignorance by the assumption of some 

 special vital force of which we know nothing. 



Although as a general rule an egg does not develop unless 

 fertilized by a spermatozoon, this is by no means always the 

 case, and many instances are known of parthenogenesis or the 

 development of unfertilized eggs. This may either be a normal 

 occurrence in the life cycle or it may be artificially induced. 



Natural parthenogenesis occurs chiefly in' insects, especially 

 amongst the aphides or plant lice. In these animals males and 

 perfect females appear only in the autumn. Fertilized eggs are 

 then laid which hibernate through the winter and hatch in the 

 spring, producing imperfect viviparous females. In these imperfect 

 females eggs are formed which develop parthenogenetically within 

 the body of the parent and give rise to fresh generations of 

 viviparous forms. This reproduction by means of unfertilized eggs 

 * See, however, the footnote on p. 189. 



