1890.] Morphology of the Blood Corpuscles. 1021 
of protoplasm reaches its ultimate size. The nucleus is at first 
granular, and its nucleolus, or nucleoli, stands out clearly; as the 
nucleolus shrinks it becomes 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 gran- 
ular 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 assump- 
tion 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 Agassiz’s law 
(Haeckel's Biogenetiches Grundgesetz). 
The blood-cells of mammals pass through the same metamor- 
phoses 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 form- 
ation of the red and white cells appears with especial 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 biood from outside. 
The non-nucleated blood corpuscles of adult mammals are 
entirely new elements which are peculiar to the class, and arise 
neither from red nor yet from white blood-cells. Their actual 
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