142 TRANSACTIONS AND PROCEEDINGS OF THE [Sess. lv. 



and an antipodal half is indicative of the embryo-sac being 

 a two-celled structure, although under now normal condi- 

 tions, no division of the protoplasm of the young embryo-sac 

 into tw^o masses takes place after the division of the primary 

 nucleus, that we would have to look at the cell-formation in 

 the antipodal half of the embryo-sac as the formation of 

 four cells, three of which are fixed to the wall of the 

 embryo-sac and separated from the remaining fourth cell by 

 definite cellulose- walls. The fourth cell would in this case 

 be a naked or primordial cell containing a nucleus. In the 

 micropylar region of the embryo-sac cell-walls appear first 

 round the synergidal nuclei, and then a very delicate mem- 

 branous wall is laid down round a third nucleus, which is 

 the nucleus of the egg-cell. Thus three nuclei at the 

 micropylar end of the embryo-sac are separated from the 

 fourth nucleus, which remains free in the protoplasm sur- 

 rounding the micropylar half of the vacuole. For the 

 reasons given above, this fourth nucleus with its protoplasm 

 must also be regarded as a primordial cell. 



If, then, this interpretation of the changes occurring in 

 the embryo-sac is correct, we must look upon the fusion 

 of the two nuclei giving rise to the definitive or secondary 

 nucleus as only the last step of a conjugation of two pri- 

 mordial cells, one from the micropylar, and one from the 

 antipodal region of the embryo-sac, and the secondary nucleus 

 as the nucleus of a newly-formed cell, occupying the centre 

 of the embryo-sac, a cell destined to act as a nurse to the 

 young embryo, to which cell I propose to give the name of 

 endosperm-cell. 



The next question to ask ourselves is this : — What in- 

 duces these two cells in the embryo-sac to conjugate ? If 

 we consider the embryo-sac a macrospore, and its contents an 

 eight-celled prothallus, we must believe two central vegeta- 

 tive cells of a prothallus capable of conjugation, with a view 

 of giving rise to tissue which will serve as a storehouse for 

 food-material. No plienomenon analogous to this can I 

 recollect. If we accept the view proposed by Ward and 

 consider the embryo-sac as containing two prothalli, we could 

 suppose very well that the micropylar prothallus gives rise 

 to two sexual cells, one being the ovum, the other the pri- 

 mordial cell lying aljove the region of the occasional septum 



