540 ME. H. MAESHALL WABD OK 



rates the two nuclei which form the tetrahedral groups, whether 

 we regard these groups, with Strashurger, as four cells of an 

 eight-celled prothallus, or as forming a separate four-celled 

 prothallus which remains sterile, &c. 



We have already seen the grounds on which rests the possible 

 view that the embryo-sac in Butomus and others arises as the re- 

 sult of two cells (products of division of embryo-sac mother cell), 

 having an extremely diffluent wall between them, gradually be- 

 coming open to one another as this wall absorbs fluid. If this is 

 so, an attempt to explain in some degree the apparently unique 

 process of conjugation between the two nuclei of the embryo- 

 sac might be suggested if we can assume that so large a cavity 

 as the embryo-sac, destined to perform important functions as 

 the embryo develops, and therefore to live vigorously for some 

 time, would be benefited by a nucleus of extraordinary vitality : 

 such a nucleus might perhaps be supposed to be formed by a 

 process of conjugation between two nuclei derived from slightly 

 different sources. 



From this point of view, it must be admitted that the em- 

 bryo-sac is either a macrospore or a joint structure formed by 

 two apposed macrospores, the success of which has become es- 

 tablished in virtue of their favourable position in the centre of 

 the nucleus. How many of the similarly formed cells of the sur- 

 rounding groups in a large nucleus are to be considered also as 

 incipient macrospores cannot be decided, and probably varies. In 

 JRosa and some Crucifers we have apparently several ; and in Py~ 

 rethrum there appear to be four as a rule, while in Antkemis we 

 have but one. There is nothing remarkable in the absorption of 

 its neighbouring cells by a successful macrospore or spore mother 

 cell, since in JPihdaria &c. similar cases exist in the sporangia. 



Postscript. 



The above had already left my hands when the paper by Vesque 

 in ' Ann. d. Sc. Nat.' Nov. 1879* was shown me : a hurried exa- 

 mination of this enables me to add a short note. 



While modifying in many respects the statement of the theory, 

 the author still commits himself to the original hypothesis. "I 

 think that the four nuclei . . . should be considered as a tetrad of 

 spores," he says (p. 263), and, further, remarks: — "I have 

 confirmed this opinion ; the morphological problem is solved : 



# « 



Sur le aac embryonnaire." 



