430 MORPHOLOGY OF THE 15L00D CORPUSCLES. 



round, and is colored darkly and almost uniformly by the usual nuclear 

 stains. This species of blood corpuscle occurs in all vertebrates, and 

 represents the genuine blood cells. According to the above description 

 we can distinguish three principal stages : (1) young cells with very 

 little protoplasm ; (2) old cells with much protoplasm and granular 

 nucleus ; (3) modified cells with shrunken nucleus, which colors darkly 

 and more uniformly. I do not know whether the first form occurs in 

 any living adult vertebrate, although the assumption seems justified 

 that they are the primitive form. On the other hand, the second stage 

 is obviously that characteristic of the Ichthyopsida in general, while 

 the third form is typical for the Sauropsida. Therefore the development 

 of the blood cells in amniota offers a new confirmation of Louis Agas- 

 siz's law (Haeckel's Biogenetiches Grundgesetz). 



The blood cells of mammals pass through the same metamorphoses 

 as those of birds ; for example, in rabbit embryos the cells have reached 

 the Ichthyopsidan stage on the eighth day ; two days later the nucleus 

 is already smaller, and by the thirteenth day has shrunk to its final 

 dimensions. 



The white blood corpuscles appear much later than the red cells, and 

 their exact origin has still to be investigated, for it has not yet been 

 determined where they first arise in the embryo ; nevertheless we may 

 venture to assert that they arise outside the vessels. The formations 

 of leucocytes outside of the vessels is already known with certainty to 

 occur in later stages as well as in the adult. The sharp distinction 

 between the sites of formation of the red and white cells appears with 

 special clearness in the medulla of bone in birds, as we know from the 

 admirable investigations of J. Denys (La Cellule, tome IV). The white 

 blood corpuscles then are cells, which are formed relatively late, and 

 wander into the blood from outside. 



The non-nucleated blood corpuscles of adult mammals are entirely 

 new elements wUich are peculiar to the class, and arise neither from 

 red nor yet from white blood cells. Their actual development was first 

 discovered (so far as I know) by E. A. Schiifer, who has given a detailed 

 account of the process in the ninth edition of Quaiu's Anatomy, and 

 has shown there a full appreciation of the significance of his discovery. 

 Unfortunately Schiifer's important investigations have received little 

 attention, Kuborn has recently confirmed Schiifer's results in an arti- 

 cle {Anatom. Anzeiger, 1890) on the formation of blood corpuscles in 

 the liver. One can readily study the i^rocess in the mesentery and 

 omentum of human and other embryos. The essential point of Schiifer's 

 discovery is that the non-nucleate corpuscles have an m^ra-cellular 

 origin, and arise by differentiation of the protoplasm of vaso-formative 

 cells. Several corpuscles arise in each cell without participation of 

 the nucleus ; they are therefore specialized masses of protoplasm, and 

 may perhaps best be compared to the plastids of botanists. I venture 

 to propose the name of blood-plastids for these structures, since the 



