172 BIOLOGY: GENERAL AND MEDICAL 



by fission, though in general it marks the end of the 

 asexual or monogenetic and the beginning of the sexual 

 or digenetic mode of reproduction. 



As in the most simple forms of life, there are no visible, 

 and probably no theoretical differences between the 

 occasionally conjoining cells, so we find that among the 

 primitive forms in which conjugation is constant, and 

 special cells are generated for the purpose, the relation 

 of these cells to one another is so close that they not in- 

 frequently descend from the same parent cell. Thus 

 a spore of the malarial parasite having attained maturity, 

 divides into a considerable number of spores which 

 develop and again divide until eventually through this 

 asexual mode of reproduction a great number of the 

 parasites is produced, all the progeny of a single cell. 

 By and by, however, a time comes when the mature 

 cells cease to sporulate as usual, and develop into mature 

 sexual forms, gametes, with which there is no further 

 development unless conjugation of two such elements 

 be permitted. 



If, in this case, it should be argued that there is no 

 certainty that the conjoining male and female elements 

 are derived from the same parent because the patient 

 may have a multiple infection, examples taken from the 

 primitive vegetable world may be given to prove the 

 case. 



In the reproduction of Eurotium repens both the 

 asexual and sexual methods may be observed. The 

 former, which is accomplished through spores, may be 

 looked upon as a kind of fission; the latter, after a definite 

 conjugation, results in the formation of a peculiar peri- 

 threcium in which a smaller number of ascospores is 

 developed. The formation of the perithrecia is not 

 easy to follow. As described by de Bary, "they begin 

 in the form of tender branches which at the termination 

 of their longitudinal growth begin to twine their free 

 ends in a spiral of four or six turns; the threads of the 

 spiral gradually approach nearer together, until finally 



