THE BLOOD AND THE LYMPH. 



717 



Fig. 294. 



new cells. New connections are also frequently formed between capil- 

 laries which arc already pervious, partly by the direct meeting of pro- 

 longations from them, partly also by the mutual connection of formative 

 cells lodged in their interstices, whence, of course, the original net is 

 rendered closer. This mode of development, so far as I have seen, ob- 

 tains in all animals, without exception, in which capillaries exist, and 

 the various objections offered to the exposition given by Schwann and 

 myself, have chiefly arisen in the notion, that every network connecting 

 the arteries and veins in embryos, is a capillary plexus. This, however, 

 is by no means the case, and consequently the circumstance, that the 

 wrongly termed capillaries of the germinal area arise after the typo of 

 the larger vessels, is not an objection of the least weight in opposition 

 to us. 



The capillaries of the lymphatic system, which may be readily traced 

 in the tail of batrachian larvce (Fig. 

 228), exhibit, essentially, precisely 

 the same mode of development as 

 those of the blood-vascular system 

 (Fig. 294), except that anastomoses 

 are rare in them, and its course is 

 more limited to the mutual apposition 

 of fusiform cells, or of cells furnished 

 with three principal processes. Ob- 

 servations are wanting with respect 

 to the larger trunks of this system, 

 although it cannot be doubted, that 

 they follow exactly the same course 

 as the bloodvessels. Engel (1. c.) has 

 lately treated of the lymphatic glands, 

 and states, that they proceed from 

 lymphatic vessels which throw out 

 buds and are much convoluted. 



The development of the blood-cor- 

 puscles is pretty accurately known, in 

 the embryo, as concerns its principal 

 stages. The first blood-corpuscles, in 

 the Mammalia and other Yertebrata 

 in general, are nucleated, colorless 

 cells, with granular contents ; they 

 are perfectly identical with the for- 



FIG. 294. Capillaries from the tail of a Tadpole: a, completed capillaries; b, cell-nuclei, 

 and remains of the contents of the original formative cells ; c, caeca! process of a vessel ; 

 d, stellate formative cell, connected by three prolongations, with three processes of pervious 

 capillaries; c, blood-globules, still retaining some granular contents. Magnified 350 dia- 

 meters. 



