DEVELOPMENT OF CAPILLARY BLOODVESSELS. 



301 



Fig. 89. 



have been observed by Professor Kblliker 1 in the tail of the very young Tad- 

 pole, at the time when it is undergoing rapid increase. The first lateral vessels 

 of the tail have the form of simple arches, passing between the main artery 

 and vein, and are produced by the junction of prolongations shot forth from 

 these vessels, with similar prolongations from stellate or caudate cells in the 

 substance of the tail (Fig. 89). Some of the latter again, coalesce with those 

 of other cells ; so that an irregular network is produced, which communicates 

 with the previously-formed trunks. The cavities of these cells and of their 

 radiations (which are at first so fine as to be almost impervious), having coalesced, 

 they begin to receive fluid from the 

 vessels, then enlarge, and finally ap- 

 pear as continuations of them. The 

 observations of Messrs. Paget and 

 Kirkes 3 on the development of blood- 

 vessels in the fine gelatinous tissue 

 conveying the umbilical vessels of the 

 embryo-sheep to the uterine cotyle- 

 dons, lead to a very similar idea of the 

 process; for here, also, there may be 

 seen chains and networks of cells of 

 various shapes, some fusiform, some 

 stellate, some round or oval with thread- 

 like prolongations, connected to each 

 other and to the adjacent bloodvessels 

 by very slender prolongations, which 

 gradually enlarge, and become filled 

 with blood from the vessels with which 

 they come into communication. Some 

 of the appearances noticed by these 

 observers, however, indicate that blood- 

 corpuscles may be formed in parts of 

 this network which have not yet come 

 into connection with the neighboring 

 vessels, and from other materials than 

 those directly derived from their con- 

 tents; for colored nucleated blood- 

 corpuscles were observed in distended 

 parts of the narrowest tubes, which 

 were connected at either extremity, 

 either with bloodvessels, or with other 

 elongated cells, by filamentous pro- 

 longations far too fine to transmit par- 

 ticles of the size of blood-corpuscles. 

 295. Some observations have been 

 recently adduced by Dr. W. T. Gaird- 

 ner, 3 which indicate that this inde- 

 pendent formation of bloodvessels and 

 of blood may take place (as John Hun- 

 ter maintained, and as many others 

 have since asserted, though without adequate evidence), to a yet greater extent. 

 The case was one in which a false membrane had been formed, within the arach- 



1 " Annales des Sciences Naturelles," Zool. Aout, 1846. 



2 "Supplement to Professor Muller's Elements of Physiology," pp. 104, 105. 



3 "Edinburgh Monthly Journal," October 1851, pp. 392-4. 



Formation of Capillaries in tail of Tadpole : a, a, 

 capillaries permeable to blood ; 6, 6, fat granules at- 

 tached to the walls of the vessels, and concealing the 

 nuclei; c, hollow prolongation of a capillary ending 

 in a point ; d, a branching cell, with nucleus and fat- 

 granules, communicating by three branches with ca- 

 pillaries already formed; e, blood-corpuscles, still con- 

 taining granules of fat. 



