102 



DEVELOPMENT OF BLOOD-VESSELS. 



tissue, and at first it is perfectly solid ; but by degrees, especially after its 

 junction with a cell, or with another prolongation, or with a vessel already 

 permeable to blood, it enlarges, and a cavity then forms in its interior. 

 Both the enlargement and hollowing out of the branch commence at the 

 point of its departure from the vessel on the one hand, and at its point 

 of junction with the cell on the other hand : the consequence of which is 

 the appearance of great irregularity in the form and size of these various 

 capillaries at their first formation. (See fig. 21.)* Of the star-shaped 

 Fig. 21.f cells described by Schwann as being 



so numerous in the substance of the 

 tail of young Batrachians, only a 

 few are developed into blood-vessels, 

 others are converted into lymphatic 

 vessels, others into nerves, while many 

 do not appear to undergo any meta- 

 morphosis. 



Plattner,| whose observations were 

 made also on the tail of the tadpole, 

 appears to have seen the formation 

 of new vessels only as effected by the 

 junction and coalescence of off-shoots 

 from previously existing vessels. He 

 observes that in this growing struc- 

 ture there may frequently be seen 

 abrupt closed extremities of capil- 

 laries, and that sometimes long 

 narrow processes may be noticed is- 

 suing from these extremities, and 

 either gradually disappearing or seen 

 uniting with other similar processes 

 from neighbouring vessels, so that two 

 such form by their union one arch 

 which gradually enlarging and becom- 

 ing permeable to blood corpuscles, con- 

 stitutes a new capillary loop. A very similar account of the jnode of 

 production of new vessels in the tail of tritons and tadpoles is given also 



* A very similar process to that above described is found by Kblliker to take place also in 

 the development of the blood-vessels of the Sepia. Entwickelungs-geschichte der Cephalo- 

 poden, pp. 82-3. 



t Fig. 21. Capillary blood-vessels of the tail of a young larval frog. Magnified 350 

 diams. After Kblliker. a, capillaries permeable to blood ; 6, fat granules attached to the 

 wall of the vessels, and concealing the nuclei ; c, hollow prolongation of a capillary, ending 

 in a point; of, a branching cell with nucleus and fat-granules; it communicates by three 

 branches with prolongations of capillaries already formed ; e, blood-corpuscles still containing 

 granules of fat. + Mailer's Archiv, 1844. 



