147 



with a microscopical power of about 1000 linear, it will appear that 

 they are rarely found to consist of a continuous and unbroken tube 

 for any considerable distance, and we shall observe that they are 

 formed of a series of moniliform parts of unequal length, the whole 

 being constructed by the junction of a linear series of minute cyto- 

 blasts, in a similar manner to that which I have described as the mode 

 of the production of similar minute vessels in the tissues of the Co- 

 rallidae. As the vessel approaches maturity, the divisions into moni- 

 liform parts become less frequent, until at last it assumes the appear- 

 ance of a tube, filled, excepting at slight and irregular intervals, with 

 a yellow gelatinous matter ; and in the complete state the gelatinous 

 line is nearly continuous and unbroken, as shown in the fragment re- 

 presented at c, fig. 4, PI. xviii. 



The vessels are not always composed of a series of cytoblasts of 

 equal diameters, nor at an equal distance from each other, but occa- 

 sionally they are constructed of small cytoblasts disposed at irregular 

 distances, along with which there are others of comparatively gi*eat 

 size, with large and very distinct nuclei, as represented at a a, fig. 3, 

 PI. xviii. In this case, when closely adjoining, they at once unite ; 

 but if short intervening spaces exist between, each cytoblast generates 

 its own portion of the tissue, which portions approximate and unite 

 together, until the whole forms a series of continuous, irregular, mo- 

 niliform vessels, as represented in fig. 3, PI. xviii.; or it sometimes 

 occurs that all the cytoblasts of the series are of the large-sized 

 description, with the distinct nucleus, as represented by fig. 5, PI. 

 xviii. The production of branches is achieved by an extension of the 

 same laws that govern the generation of the main trunks of the ves- 

 sels. A single cytoblast is generated upon, or becomes attached to, 

 the side of a vessel. Membranous structure is detached from its 

 under side, in accordance with the laws that I have previously 

 described, and a short cylinder of vascular structure is produced, 

 bearing upon its summit the large spherical cytoblast from which it 

 has emanated, and in proportion as this organ is endowed with pro- 

 ductive power, so we have the branches either long or short. While 

 in a state of vigorous action the cytoblast appears to retain its sphe- 

 rical form, original diameter, and distinctly nucleated appearance, as 

 represented in a, fig. 4, PI. xviii.; but as it declines in vigour it de- 

 creases in size, assumes an oval shape, and loses the nucleated ap- 

 pearance, (fig. 4, b) ; and this diminution of its diameter continues 

 gradually to take place, until, at the period of the cessation of its 

 productive power, it is reduced to the ordinary diameter of the vessel 



