S3° FORMATION OF CAPILLARIES. [SECT. 222. 



for the vessels is being constantly supplied by the formation of 

 new cells. New connections, also, are frequently formed between 

 the already pervious capillaries, partly from the direct coalescence 

 of processes, partly from connections with interjacent formative 

 cells ; so that by this means, of course, the primitive network 

 becomes denser. — This mode of formation holds good, without ex- 

 ception, as far as I have seen, for all creatures in which capillaries 

 occur; and the objection raised in various quarters against the 

 exposition of Schwann and myself, have chiefly resulted from the 

 circumstance, that observers have regarded every network uniting 

 the arteries and veins in the embryo as consisting of capillary 

 vessels. This, however, is by no means the case ; and, accordingly, 

 it is no contradiction whatever to our views, that the so-called 

 capillaries of the germinal area, which are no capillaries at all, 

 should arise after the type of the larger vessels. 



The capillaries of the lymphatic system, which can be readily 

 traced in the tail of the batrachian larva (fig. 205), have exactly 

 the same development in all essential points as those of the system 

 of blood-vessels (fig. 215), except that here anastomoses are rare, 

 and the mode of formation is more limited to the deposition of 

 cells in linear series; the cells being either fusiform, or having 

 three chief processes. Observations are wanting on the larger 

 trunks of this system, yet it cannot be doubted that they agree 

 entirely with the blood-vessels in their development. Engel has 

 recently treated of the lymphatic glands (1. a), and stated that 

 they arise from lymphatic vessels which give out buds, and become 

 variously convoluted. 



The development of the blood-corpuscles in the embryo is pretty 

 accurately known in its chief stages. The primitive blood-cor- 

 puscles in mammalia, and the vertebrata in general, are nucleated, 

 colourless cells, with granular contents, which are perfectly iden- 

 tical with the formative cells seen in all parts of the young embryo. 

 The first corpuscles are formed in the rudimentary heart and great 

 vessels (in some places very early ; in others, somewhat later) by 

 separation from each other of the cells in the centre of these 

 organs, and this separation results from the formation of fluid, the 

 primitive blood-plasma, between them. From these colourless 

 cells arise the first coloured blood-corpuscles ; they lose their gra- 

 nules, and become filled with hcematine ; but retain their nucleus. 



These coloured, nucleated, primitive blood-cells are spherical, 

 more intensely coloured, and larger than the blood-corpuscles of 

 the adult, but in all other respects, are similar to them. In a 



