17 



and becomes, as already slatcMl, the ciiil)r)-o-sac cell. At 

 this stage it is observed (Fig. 15) deeply embedded in the 

 nucellus, l)ing at tiie bottom of a channel formed by the 

 decay of the three oxerlying cells of the tetrad. The top 

 of this funnel-shaped tube is capped by a disorganised 

 mass of cytoplasm dern'ed from the deleted tetrad cells. 



During these changes the integuments of the ovule 

 or megasporangiuni become differentiated, and by the 

 tunc the tetrad stage is reached the inner one is 

 .some O-H cells long, the outer 2-4 cells; each is two rows 

 of cells wide, and remains so throughout the whole course 

 of development up to the time of fertilisation. 



MEGASPORE OR EMBRYO-SAC. 



The first mitosis of this large cell appears to be 

 Koernicke's reduction-division, i.e., the mitosis in which the 

 number of chromosomes in the nuclei of each of the two 

 resulting cells is reduced by one-half, and indicates the 

 inception of the gametophyte or sexual generation.* The 

 number of chromosomes found by him in the nuclei of the 

 sporoph}'te or asexual generation, was generally iC, that in 

 the gametoph)'te nuclei 8. Unfortunately my preparations 

 lia\e not shown the chromosomes clearly enough to count. 

 In the first division of the microspore mother-cell nucleus 

 he gives the same number (8), and this I have found to 

 be the case (Fig. lo). Golinsky-^ gives for Sccalc cereale, 

 the rye, 16 and 8 chromosomes for the respective 

 generations. 



Germination of the embryo-sac follows the usual 

 course in the first three divisions of its nuclei. The 

 daughter nuclei of the first mitosis (Fig. 18) take up 



*Meiotic i.s now being somewhat generally applied to the reduction 

 di.'ision in either a niegaspore or microspore mother-cell; pre-meiotic 

 and post-meiotic denoting the mitoses preceding and succeeding that 

 phase. 



