126 



we observe that the horny structure continues to abound in the vesi- 

 cular bodies in all their earlier stages of growth ; from which it would 

 appear that its office is not confined to the production of the first 

 stratum of calcigerous cells only, but that it is destined to originate 

 perhaps the greater part, if not the whole, of the calcigerous cells of 

 the shell, and this appears the more probable, as we may now observe 

 that the membrane has attained a higher stage of organization) mi- 

 nute vascular tissue being frequently observed embedded and ramify- 

 ing amid its structure, and oftentimes projecting from its torn edges. 

 These vessels are very minute, none of those which I have observed 

 exceeding T4 ^ 00 . of an inch in diameter. If the uncovered portion 

 of the membrane nearest to the old lip of the shell be examined with 

 a power of 500 linear, these minute vessels may be seen in the course 

 of formation, presenting the appearance of long ramifying lines of 

 exceedingly minute gelatinous molecules, closely adhering together in 

 a single series, and frequently appearing cylindrical and tubular in one 

 one part, while in another they have quite a moniliform appearance, 

 (PI. xiv. figs. 1 & 2, a). This production of vascular structure by the 

 arrangement of minute molecules or cytoblasts in single series, I have 

 frequently observed in the membranous tissues of the Corallidae.* 

 The occurrence of perfectly developed vascular tissue, at the advanced 

 stage of the formation of the new lip of the shell which I have 

 described, is exactly what might have been expected to take place, 

 as, according to the best authorities, cellular cartilage in the higher 

 classes of animals is not penetrated by blood-vessels ; but when it 

 has become filled with bony secretions, they are then found permeat- 

 ing its structure in every direction. In a similar manner, in the early 

 stage of development of the shell-structure, we may naturally expect 

 the same law to prevail, and accordingly we find the newly-formed 

 cartilage or membrane of the shell abounding in the rudimentary ve- 

 sicles of the cytoblasts, while not even a trace or indication of the 

 vascular structure is to be seen in those parts which are nearest to the 

 outer margin of the new lip. 



The linear aggregation of cytoblasts, described as producing the 

 minute ramifying vascular tissue, is not the only case of this mode of 

 arrangement which we observe in the young lip of the shell, as we 

 find irregularly dispersed over the surface of the membrane — and fre- 

 quently floating in the surrounding water in w r hich it has been im- 

 mersed for examination — cylindrical bodies, of equal diameters in all 



* See^' Philosophical Transactions,' part 2, 1842, page 220, pi. 17, fig. 2. 



