562 THE AMERICAN NATURALIST. [VOL. XXXIX. 



There are two types of parthenogenesis in plants : (i) that in 

 the thallophytes where there is no sporophytic generation, and 

 (2) that in higher forms when the life history is complicated by 

 an alternation of generation. We know nothing of the cytologi- 

 cal conditions in the first group including such types as Chara 

 crinita, Cutlaria, some species of Spirogyra and Zygnema and 

 numbers of the lower Chlorophyceae and Phaeophyceae whose 

 motile gametes will germinate like zoospores should they fail to 

 conjugate with one another. But since there is no reason to 

 suppose that there are reduction phenomena at gametogenesis, 

 the unfertilized gamete is fully prepared with respect to the 

 number of chromosomes to continue the parent stock. Dictyota 

 must be excluded from this list since the parthenogenetic devel- 

 opments here are abortive. In the second group parthenogene- 

 sis is likely to prove to be the result of a suppression of reduction 

 processes during sporogenesis by which a gametophyte genera- 

 tion retains the sporophyte number of chromosomes and in 

 consequence is prepared to dispense with sexual processes in the 

 development of a new sporophyte. Parthenogenetic develop- 

 ment in animals seems to be similar in its essential cytological 

 features to parthenogenesis and apogamy in plants. There may 

 be a suppression of reduction processes somewhat comparable to 

 that discussed above, which takes place, however, at the time of 

 gametogenesis, whereby the egg nucleus retains the number of 

 chromosomes characteristic of the parent. Or, through a fusion 

 with the nucleus of the second polar body the egg nucleus is 

 brought back to the normal condition with respect to the num- 

 ber of chromosomes of the parent stock. We cannot, however, 

 consider in detail the forms of parthenogenesis in animals. They 

 have been recently treated by Blackman (: O4b) in comparison 

 with conditions in plants. 



Apogamous developments which involve wholly or in part 

 other elements than gamete cells and nuclei are likely to be 

 established in a number of groups of the thallophytes. The 

 author has long believed that the cystocarps of some of the 

 Rhodophycese develop apogamously, basing his conclusions on 

 certain general peculiarities of the group and more particularly 

 on a study of Ptilota (Davis, '96). Three species of this genus 



