i 9 o8] YAMANOUCHI— APOGAMY IN NEPHRODIUM 293 



feature in the life-history; and therefore the problem of alternation 

 must be settled by an appeal to evidence other than that derived from 

 the facts of mitosis. 



Strasburger (68) studied apogamy in Marsilia from the cytologi- 

 cal standpoint. Previous to his work, parthenogenesis in Marsilia 

 had been described by two observers. Almost ten years ago Shaw 

 (62) found normal parthenogenesis, as it was called by the author, to 

 be of frequent occurrence. He also isolated megaspores of Marsilia 

 Drummondii from microspores before the sperms matured, and over 

 50 per cent, of the isolated female prothallia produced embryos, while 

 not more than 69 per cent, of those which were mixed with male 

 prothallia produced embryos. Four years later Nathansohn (48) 

 induced parthenogenesis in Marsilia vestita and M. macro,. He 

 found that it was possible to stimulate the egg cell to a parthenogenetic 

 development by exposing the germinating megaspores to a temperature 

 of 35 C. for 24 hours, and allowing them to continue their development 

 at a temperature of 27 C. As a result, about 7-12 per cent, of the 

 spores gave rise to parthenogenetic embryos. Under lower tempera- 

 tures the egg developed an embryo only after fertilization. No nuclear 

 conditions were noted by Shaw or Nathansohn. 



In parthenogenesis in seed plants, a reduction of chromosomes 

 does not occur in the formation of the embryo sac, so that the egg 

 nucleus contains the diploid number, which naturally obviates any 

 necessity for the act of fertilization. Considering parthenogenesis in 

 seed plants, there arises naturally a question as to how the egg nucleus 

 in Marsilia, which usually establishes a new sporophyte after a normal 

 act of fertilization, could have developed parthenogenetically into the 

 sporophyte. A clear explanation of this question from the cytological 

 standpoint was made by Strasburger in his recent paper (68) . 



Strasburger states that the classification into species in Marsilia 

 ■cannot always be relied upon. He finds that megaspores of Braun's 

 Marsilia Drummondii develop embryos habitually by parthenogenesis, 

 and that in three species — M. vestita, M. aegyptica, and M. quadri- 

 jolia — embryos are formed only after fertilization. 



In Marsilia Drummondii he finds that the nuclei of the prothallia 

 in the megaspores contain 32 chromosomes, the diploid number, as 

 found in the root tips and other vegetative structures of the sporophyte. 



