Osyris, Loranthus and Viscum. 187 



Very generally it is confined to the nucleus, and so far as I know, the first 

 notice of its continuous exsertion was given in Santalum. 



Various considerations induced me to pay considerable attention to the 

 origin and subsequent relations of this sac in Osyris and Santalum, but I 

 cannot say that the observations have appeared to~me altogether satisfactory. 

 I am inclined, however, to believe that in their earliest stages they do not differ 

 from the ordinary fashion of sac ; neither is the extension of a single cell a 

 modification, so far as my experience goes, generally confined to albuminous 

 seeds, nor its degree particularly remarkable ; it is in the protrusion and 

 extension backwards that the anomalies consist. 



In Osyris none of the means at my disposal enabled me to detect the sac in 

 the placenta before its presence in the ovulum was ascertainable ; nor did I 

 detect any such different degrees in its extension backwards as might have 

 been expected. 



In Santalum I believe the sac is developed from the interior of the nucleus, 

 first in an anterior, subsequently in a posterior direction ; and this, connected 

 with its apparent limitation at a very early period to the nucleus, and the 

 non-extension of the sacs of the ovula in Osyris, which are barren, point out, 

 I think satisfactorily, that what I have called the nucleus is in reality the 

 ovulum : otherwise it might have been, it appears to me, an open question 

 whether the placenta itself was not the ovulum (analogous to those which 

 contain more than one embryonary sac), to the nucleary forms of which it has 

 considerable similarity in the cellularity of its apex. It must be confessed, 

 however, that apparently formidable objections to this assumption would exist 

 in the want of a common line or point of fecundation, and more importantly 

 still, of correspondence in direction of the sac and supposed nucleus. 



The extension backwards of this sac is also, I believe, now pointed out for the 

 first time, unless the apparatus, to which M. Decaisne is said to attribute the 

 function of a chalaza, be something of the same nature. It first passes upwards 

 until it reaches the axis of the placenta, or nearly so ; it is then deflexed : its 

 presence appears always to be connected with a slight browning of the tissues, 

 with which it is in contact : subsequently it reaches a considerable distance 

 below the base of the placenta. In Santalum the extension, especially the 

 placental portion, presents an irregular surface, and throughout, but most 



vol. xix. 2 c 



