No. .325] SEXUAL REPRODUCTION IN ALGJE 



515 



chromosome reduction there is always the possibility of 

 an intercalated sporophytic phase in the life history," the 

 development of which will involve the mitoses of nuclei 

 that cany twice the number of chromosomes character- 

 then consider in the order of the r!ilorophycea\ Pluro- 

 phycea> and Rhodophyeeae some of the recent cvtologieal 

 research which has made much clearer the significance 

 of the life histories of certain important types. 



There are in the life histories of a number of the green 

 alga? certain outward signs that would lead one to sus- 

 pect that the period of chromosome reduction in these 

 forms takes place just previous to or during the germi- 

 nation of the zygospores or oospores. Thus the forma- 

 tion of several zoospores within the zygote of Ulothrix 

 and Hydrodictyon, and of four zoospores in the oospore 

 of (Edogonium at once excites suspicion that the first 

 mitosis in these sexually formed cells is a reduction divi- 

 sion. Still more striking is the phenomena described in 

 the zygospores of the Conjugal es where the fusion nu- 

 cleus by two mitoses gives rise to four daughter nuclei 

 three of which in Spirogyra and two of which in the des- 

 mids Closterium and Cosmarium break down. Such be- 

 havior, involving a sacrifice of protoplasmic structure 

 and energy, is not easily understood except that it be 

 related to important events of ontogeny and phylogenv. 

 Any one familiar with the cytology of spore formation 

 in the higher plants or with oogenesis in animals would 

 at once suspect that the zygospore of the Conjugates was 



That this is really true hasten shown by recent work 

 of Karsten 8 who has followed the chromosomes of Spiro- 

 gyra jugalis through the two mitoses within the zygo- 

 spore. Karsten in agreement with Bergh> reached' the 

 conclusion that the chromosomes in the resting nucleus 



