REPRODUCTION 193 



themselves go on to adult development, we pass to forms 

 in which the conjoining cells must come from different 

 individuals. Thus in Mucor mucedo the formation of 

 zygospores follows the conjugation of cells from different 

 colonies, never from the conjugation of cells from the 

 same colony. The spreading mycelia of this mould, 

 reaching the spreading mycelia of another colony may 

 or may not conjoin according to the nature or conditions 

 of the two. Thus when carefully examined it is found 

 that they can be separated into two strains represented, 

 respectively, by the signs + and . Colonies of the + 

 strain will not conjoin; colonies of the strain will not 

 conjoin, but 4- and will conjugate and form zygospores. 

 The actual steps in zygospore formation are not difficult 

 to follow. The gametes are clavate terminal enlarge- 

 ments of the mycelial threads. As they come together 

 the tips flatten and soon a short cell, the real conjugat- 

 ing cell or gamete, makes its appearance through the for- 

 mation of a line of cleavage appearing a short distance 

 from the point of contact. 



The original mycelium is now known as a suspensorium, 

 the conjoined cells as gametes. There may be one or 

 two zygospores according to the subsequent develop- 

 ment of one or both of the gametes. In cases where 

 actual fusion of the gametes takes place, there can be 

 but one zygospore; where a transfer of substance from 

 one to the other occurs without fusion, two may form. 

 The zygospores become surrounded with a thick mem- 

 brane which, under favorable conditions, ruptures to 

 permit the escape of the growing hypha. The advan- 

 tage of conjugation in these cases is not clear, for the 

 formation of spores surrounded by dense membranous en- 

 velopes azygospores may occur without conjugation. 



Thus in examining the lowly forms of life, both animal 

 and vegetable, we are struck by the growing tendency 

 toward amphimixis or the digenetic mode of reproduc- 

 tion, the importance of which is shown by the ingenious 

 means taken to bring it about, and the final disappear- 

 13 



