490 [March 19, 



the structure of the vertebrate blood-disk is important. They 

 seem to warrant the inferences drawn in the two following para- 

 graphs : 



1 . The exact identity of the appearances produced in the blood- 

 disks of the ovipara with those observed in the mammalian corpuscles 

 lends strong support to the view that these corpuscles are homolo- 

 gous as wholes; and that the mammalian blood-disk is not the 

 homologue of the nucleus of the coloured corpuscle of the ovipara, 

 as was conceived by Mr. Wharton Jones. 



2. The observations likewise lead to the belief that the envelope 

 of the vertebrate blood-disk is a duplicate membrane ; in other words, 

 that within the outer covering there exists an interior vesicle which 

 encloses the coloured contents, and, in the ovipara, the nucleus. 



Dr. Hensen* of Kiel had already in 1861 convinced himself, from 

 wholly different observations, that the blood-corpuscles of the frog 

 possess such a structure. On this view the blood-corpuscle is 

 anatomically analogous to a vegetable cell, and the inner vesicle 

 corresponds to the primordial utricle. 



The present observations indicate, by direct proof, a duplication at 

 only one or, at most, two points in the blood-disks of mammals and 

 birds. Nevertheless certain appearances, occasionally observed, favour 

 the notion of a complete duplication (fig. 1, 6). 



The admission of this hypothesis, however, scarcely removes the 

 difficulties sufficiently to permit a tenable explanation to be offered 

 of the appearances described in this paper. Yet, as it may prove 

 suggestive to some other inquirer, I will not suppress what appears 

 to me the explanation least open to objections. It might be con- 

 ceived that the cells enlarged by imbibition, until at length the less 

 distensible inner membrane gave way, and permitted an extravasation 

 of a portion of the cell-contents between it and the outer membrane, 

 its own continuity being in the meanwhile instantaneously restored 

 by cohesion of the ruptured borders f. In this way a microscopic 



* Zeitschrift fur Wissensch., Zoologie, Band xi. p. 263. 



t In the same manner as a soap-bubble when bisected, instead of collapsing, 

 forms, in virtue of the adhesiveness and fluidity of its envelope, two new and per- 

 fect bubbles. That the cell-wall of the blood-disk possesses some such endow- 

 ment seems highly probable. I have on several occasions witnessed, after adding 

 magenta, the total extrusion of the nucleus, both in the frog and in the newt, 

 without the least collapse of the corpuscles. 



