542 DEVELOPMENT OF THE SIMPLE TISSUES, B\ r S. STEICKEE. 



can be observed as soon as circulation has commenced, the 

 formation of buds is so obvious that it cannot be overlooked by 

 any careful observer. 



The vascular walls send out processes which augment in 

 thickness, and unite with the processes of other vessels, or with 

 other vessels directly ; as soon as these become hollow the 

 communication is established. It is moreover probable that 

 in the tail of Tadpoles free cells acquire processes, and attach 

 themselves to a vessel in order to play the same part which, 

 in accordance with the above description, 'is played by the 

 vascular processes. My observation that in the tail of the 

 Tadpole blood-containing fusiform cells, closed at both extre- 

 mities, occur, has recently been corroborated by J. Arnold. 

 These observations render it probable that even in the tail of 

 the Tadpole an endogenous development of blood corpuscles 

 takes place. It is moreover rendered certain, by researches 

 that have been for some time past conducted in my laboratory, 

 that blood can originate endogenously in the so-called vascu- 

 larizing inflammatory foci (vascularisirenden Entzundungs- 

 heerden) the walls of the cells becoming converted into vascular 

 walls. No other mode of the new formation of vessels has as 

 yet been observed. 



Originally all vessels, whether they subsequently form the 

 heart, arteries, or veins, are constructed similarly to the capilla- 

 ries ; that is to say, they have only a single nucleated wall, 

 and this wall in the embryonic condition is composed of 

 embryonic cell substance or protoplasm. The increased com- 

 plexity of structure subsequently acquired by the heart, arte- 

 ries, and veins, is the consequence of a secondary process in 

 the external wall of the original tubular system, of which we 

 have at present no information. The endothelia of the heart, 

 arteries, and veins have thus the same genetic importance and 

 value as the walls of the capillaries. 



Inasmuch as a system of brown lines can be brought into 

 view by the action of nitrate of silver, exactly resembling 

 those of completely developed capillaries, and continuous with 

 the brown striae of cement of the endothelia both of the 

 arteries and of the veins, we must admit that the striae of 

 cementing substance must have formed subsequently through- 



