148 



it has generated, and finally forms a ccecoid termination to it. By the 

 production of branches in this way from all parts of the originally 

 small and widely separated centres of vascular tissue, the whole be- 

 comes ultimately united, and forms one large and intricate plexus ; 

 for it may be frequently observed in the young basement membrane, 

 that while some of the groups of vessels are isolated, there are 

 patches situated near each other, which, although each may have freely 

 anastomosed with the members of its own group, yet they are con- 

 nected with each other by one or two points of junction only, while, 

 on the contrary, in the more advanced stage of the tissues, isolated 

 vessels or centres of vascularity are rarely to be detected in the inter- 

 stices of the general plexus. 



The vascular tissue, which I have described above, is not peculiar 

 to the new membrane of the wounded parts of the shell, but it exists, 

 I believe, in the same relative situation on all parts of the basement 

 membrane of the uninjured periostracum, as it may be frequently 

 observed when that organ is sufficiently thin and transparent to permit 

 of distinct vision with high microscopical powers, and especially in 

 the young portions near the edge of the shell. 



I have not had the opportunity of examining either human false 

 membrane in a sufficiently early state of development, or human foetal 

 tissues, to be enabled to say that the production of vascular tissue in 

 our own species is in accordance with the facts which I have just de- 

 scribed ; but I have little doubt that ultimately such will prove to be 

 their origin, for Bowman has noticed corpuscles in the coats of capil- 

 lary blood-vessels, in the sheath of nerve, and in the substance of ten- 

 don, but believes them to be " the nuclei of cells, from which these 

 several structures have been originally developed." 



The arrangement of the cytoblasts in linear series is probably due 

 to an electrical or polar action among themselves, each molecule ap- 

 pearing to have a distinct axis that determines the position in which 

 one approaches the other. Thus in the compressed discoid cytoblasts 

 of the animal membranes of the coral, Millepora alcicornis, the polar 

 axis appears to pass from certain points in the circumference, through 

 the centre of the body, midway between the two broad surfaces ; thus 

 inducing an arrangement of the disks edge to edge : while in the disks 

 of the human blood, the polar axis appears to pass from centre to cen- 

 tre of each of the broad disks of the corpuscles, or in a direction ex- 

 actly the reverse of the cytoblasts of the coral : thus inducing a linear 

 arrangement of the blood-disks, with their broadest surfaces in con- 

 tact, or after the manner in which a pile of coin is packed, and which 



