14tf 



beautiful cytoblast, which, having been originally seated on the base- 

 ment membrane, had projected a cylindrical tube of membranous 

 structure from the under side of its margin, to such an extent as fre- 

 quently to elevate it fifteen or twenty times the length of its own di- 

 ameter, from the surface of the membrane upon which it originally 

 reposed. These sacculated bodies are almost always furnished with 

 extremely minute vessels, that occasionally give off short branches, 

 which are sometimes observed to terminate in a linear series of three 

 or four cytoblasts. 



I have alluded to the description of these structures somewhat at 

 length, as it will be seen that the vascular tissues on the new basement 

 membrane of the wounded shell are intimately connected in their 

 origin and structure with those of the Corallidae, and that in conjunc- 

 tion with them they will afford interesting demonstrations of the 

 mode of generation of the primary vascular tissues, not only in ani- 

 mals comparatively of a low degree of organization, but also, it is to 

 be hoped, in those of the higher warm-blooded classes, as I have ob- 

 served these primary vessels amid the cartilaginous fibres of a portion 

 of the foetal skull of an infant, which had been deprived of its carbo- 

 nate of lime by maceration in dilute hydrochloric acid, and also in 

 the somewhat similar cartilaginous structure in a case of mollities 

 osseum, at St. Thomas's Hospital, described by Mr. Samuel Solly. 



Let us now return to the basement membrane of the wounded 

 shell. If, with a microscopic power of 500 linear, we examine this 

 tissue in the earliest stages of its development, and before it has 

 closed in the centre, there are usually but very slight traces of vascu- 

 larity to be found. A few simple but very slightly branching vessels 

 are to be seen dispersed, frequently at considerable distances from, 

 each other. In this stage of their existence they do not appear to be 

 a part of one great plexus buried in the substance of the membrane, 

 of which portions only are to be seen, but are truly separate centres 

 of primary vascular structure ; for if we use a high magnifying power, 

 1000 or 1500 linear, and focus the structures with great care, we may 

 readily convince ourselves that they are not embedded in the substance 

 of the basement membrane, but are produced upon its surface, and 

 are but slightly adherent to it, for it frequently occurs that branches 

 are seen projecting from its plane ; and at the torn edges of the mem- 

 branes also they are often seen extending considerably beyond the 

 margin, and in this state are frequently terminated by a large and 

 well-defined cytoblast. 



If we examine these detached patches of young vascular tissue, 



