Osyris, Loranthus and Viscum. 175 



cellular tissue, which reaches to a considerable length before the more con- 

 densed structure of the embryo becomes visible. 



As the cellularity of the original sac increases, all traces of its original 

 structure disappear, and it becomes a cellular mass, divided completely into 

 as many parts as there are component cells. (Tab. XVII. figs. 10 and 11.) 



During these changes the posterior extensions of the sacs have become 

 longer and more divided, and they penetrate to a considerable distance below 

 the placenta into the white cellular substance constituting the ovarium, form- 

 ing extensive relations of superficies with it. (Tab. XVII. fig. 8.) 



It is perhaps unnecessary to trace the development further. I have ascer- 

 tained the existence of the vesicle, in the state I have just mentioned, when 

 the embryo is half-developed. It appears, perhaps, throughout to have rela- 

 tions to the lax cellular mass of the embryo a good deal similar to those it 

 originally bore to the grumous molecular contents of the embryo-sac. 



It will be seen from this that my present statements regarding Santalum 

 diflFer considerably from my previous ones, with regard to the apex of the sac, 

 its posterior extension, the subsequent separation of continuity, and its rela- 

 tions with the boyaux. 



With regard to the first point of difference I have nothing to offer ; that 

 which I have now described as the appearance of the mature apex requiring 

 no nicety of observation. To the extension posteriorly and the interruption 

 of continuity I was led by Osyris ; otherwise, from the parts remaining in 

 contact, it is liable to be overlooked. The last correction has resulted from 

 very extensive repetition (for the " blending" is occasionally apparent), and 

 from improved means of observation*. 



* I may here mention, that the observations on Santalum, Loranthus and Viscum were made almost 

 exclusively with one of Mr. Ross's compound achromatic microscopes, the object-glasses of which were 

 supplied to me in 1 836, and have respectively |th and ^^i\i inch focal distances. Verification, when ne- 

 cessary, was made by excellent triplets of various powers by the same eminent optician. I have also had 

 the advantage, as occasions offered, of examining some of the minuter points by the superb Rossian 

 microscope of Mr. Grant, to whom I am indebted for direct proof of the engagement of the pollen tubes 

 in the apex of the embryo-sac. 



My compound microscope, though now of some years' date, is fully equal to any triplet in my pos- 

 session in defining and penetrating powers ; and as I have from repeated comparison acquired confidence 

 in it, and as it possesses such obvious advantages over any modification of the simple microscope, I sel- 

 dom now use any other. The observations on Osyris were made almost entirely by simple microscopes ; 

 all the minuter points being observed under excellent triplets of 5'gth and j^th inch focal distances. 



