Lecture XX VI II. 257 



mitoses, represent halves of chromomeres of the original thread, 

 but are these segments themselves intact. Hence the chromomeres 

 in the resulting nuclei may not be equivalent. 



The two nuclei formed by the reduction-mitosis immediately 

 undergo a second division. This mitosis conforms to the ordi- 

 nary procedure of somatic nuclear division, except that in the 

 differentiation of the chromosomes the reduced number of 

 chromosomes appears. The nuclei then of the second mitosis of 

 the sporogenous cells contain this reduced number, or if we indicate 

 the number of chromosomes in the somatic cells as 2.x these nuclei, 

 which, as we know, become the nuclei of the spores, contain x 

 chromosomes. The former cells and nuclei are termed diploid in 

 contradistinction to those which contain the x number of chromo- 

 somes and are called haploid. 



When the spore germinates, a series of mitoses is initiated which 

 preserves the x number of chromosomes and the cells of the gameto- 

 phyte are thus haploid. This series terminates with the production 

 of the sperms and ova which, of course, are also haploid. Fertilisa- 

 tion brings the two haploid nuclei together in the oosperm, and 

 initiates the diploid series which is the sporophyte. The diploid 

 phase then persists till reduction in the spore- mother-cells re- 

 establishes the haploid phase, which continues throughout the 

 gametophyte. 



In all the Archegoniates and in the Seed-Plants the reducing 

 division normally takes place in the spore- mother- cells, and the 

 tetrad cell-division characteristic of spore- formation is thus as- 

 sociated with the reducing mitosis which initiates the haploid 

 phase. The cell-division producing the megaspore of Seed-Plants 

 is never a characteristic tetrad division the four cells resulting 

 form a linear series ; and it is just in connection with the produc- 

 tion of the megaspores that an irregularity in the occurrence of the 

 reducing mitosis has been found. Instead of the reducing division 

 occurring in the mother- cell, it is often postponed till the first 

 division of the megaspore (embryo-sac). 



In Polysiphonia as we might expect from the formation of the 

 tetraspores reduction takes place at their origin, and the gametophyte 

 resulting from their germination is haploid. Haploid nuclei char- 

 acterise its gametes. The oosperm is evidently diploid and pro- 

 duces by its nuclear and cell- divisions the diploid sporophyte, 

 which by a reducing mitosis produces the haploid tetraspores. So 

 the two phases correspond with the limits of the alternating sporo- 

 phyte and gametophyte. 



Evidently a different state of affairs must obtain in a plant like 



17 



