570 Transactions of the Societtj. 



may be studied at once under the Microscope, or mounted on a 

 slide as a permanent preparation. 



It may with equal facility be rendered transparent by alcohol 

 and an essential oil, and mounted in balsam or copal varnish, but 

 it then possesses all the disadvantages of a balsam preparation. 

 Under all circumstances, the membrane must be clarified before it 

 is excised from the rings, to prevent unequal contraction. It is 

 easily excised by running the edge of a knife round the outer rim 

 of the inner ring, and having prepared a slide previously with a 

 drop of glycerine upon it, the disk of membrane remains in place 

 when applied to it ; the glass cover may then be put on and sealed, 

 as we do it, by hot sealing-wax dropped round the edges, and 

 trimmed with a hot wire while the whole is compressed by a paper- 

 clip. 



We have thus a preparation mounted in glycerine, in which no 

 undue distension has taken place, to whose surface no injury has 

 been done during the whole course of preparation, and whose 

 progress at every stage could be examined under the Microscope 

 without damaging it. Moreover, when mounted in glycerine the 

 blood leaves the vessels when the disk is excised, and is washed 

 away at the edges with the excess of glycerine, so that all the 

 vessels appear as rigid, hollow tubes, the thickness of whose walls 

 and the joints and nuclei of the cells composing them, can be 

 equally well seen by the silver and pyrogallate of iron processes we 

 have used. 



As an admirable little review of the opinions already arrived at 

 by difterent observers on the question of the development of blood- 

 vessels, has lately been given by Dr. George Thin in ' The 

 Quarterly Journal of Microscopical Science ' for July, 1876, we 

 think it inadvisable to lengthen out this paper by any recapitula- 

 tion of them. With regard to even the latest views. Dr. Thin 

 states : — " The conclusion to which I have therefore come is, that 

 the cellules vasoformatives of Eanvier are spaces in the omentum, 

 to which, I submit, the term ' cell ' is not applicable. The develop- 

 ment of blood-vessels takes place by an escape, first of fluid, and 

 finally of the formed elements of the blood from the vascular 

 system into these spaces. The establishment of the blood current 

 is speedily followed by the formation of a membranous wall around 

 the current, which is impermeable for an injection mass or the 

 blood, and the process is complete." 



We are careful to give Dr. Thin's views in his own words, as 

 they are the latest, to our knowledge, which have appeared in 

 English. They are opposed to the views of all previous observers, 

 and they are equally opposed to all the facts we have ascertained 

 and are about to state in this paper. Indeed we fail to understand 

 how, if he has used the silver process, he has overlooked the fact 



