No. 525] SEXUAL REPRODUCTION IN ALG2E 



521 



dies of the first mitosis are multipolar and the chromo- 

 somes are distributed irregularly to a cluster of small 

 nuclei. It would seem then either that the sperm brings 

 to the egg a directive element in the form of an aster, 

 which gives polarity to the fusion nucleus, or that in the 

 absence of fertilization the egg is unable to develop the 

 mechanism necessary for a normal mitosis. 



It is the purpose of this paper to consider the nuclear 

 phenomena of sexual reproduction among the algae in the 

 broadest sense, and this must involve the consideration 

 of any sporophytic generation when present. It has be- 

 come clearly established by the cytological research of 

 recent years that the sporophytic phase in a life history 

 is a period during which one of the final ends of a sexual 

 nuclear fusion, the intimate association of the sets of 

 parental chromosomes, is delayed and in consequence the 

 mitoses of this period deal with double the number of 

 chromosomes present in the sexual plants. Williams 

 found in the case of Dictyota that the mitosis which cuts 

 off the tetraspore mother-cell presented the same large 

 number of chromosomes as the first division of the egg, 

 i. e., twice the number characteristic of the sexual plants. 

 The inference is clear that the vegetative mitoses 

 throughout the development of the asexual plant must be 

 sporophytic in character, and that this generation must 

 have developed from the fertilized egg. 



With respect to the two mitoses within the tetra- 

 sporangium, the main events are evidently those char- 

 acteristic of a numerical reduction of the chromosomes. 

 There is a stage of synapsis in which a long thin spirem 

 is found closely coiled in knots against the nuclear mem- 

 brane. Following this comes a loosening up of the spirem 

 and the differentiation of sixteen chromosomes, the re- 

 duced number. The form of the chromosomes, which are 



with the peculiarities of the heterotypic mitosis that 

 these structures are really pairs of sporophytic chromo- 

 somes (bivalent chromosomes) in close association. 



