THE HISTOLOGY OF THE BLOOD OF.LARVA OF ZLEPIDOSIREN PARADOXA, 463 
cells. ‘he outer cells, later, form the capsule; the central cells give rise to cellular 
trabecular tissue, which in part becomes the connective tissue of the fully formed organ, 
in part gives rise to free cells. 
(15) In the earlier stages the splenic vein is full of mononuclear cells, with nuclei of 
the leucocyte type, identical with those in the blood at stage 32, before the spleen has 
differentiated. There are also in the vein a few polymorphonuclear cells, and these are 
in large numbers in the spaces of the trabecule. 
(16) In later stages the spleen pulp contains fist basophile cells, with the leucocyte 
type of nucleus. In some parts the remains of the basophile cellular trabecule are seen 
containing nuclei of the same type; second, similar cells with concentric fibrillation 
of the basophile protoplasm ; thord, cells with nuclei of the erythroblast type and con- 
eentric fibrillee in the protoplasm; fourth, cells with nuclei of the erythrocyte type, 
and yellow granules in the ringed cell-body ; fifth, mature erythrocytes; sixth, mono- 
nuclear leucocytes with basophile protoplasm ; seventh, polymorphonuclear leucocytes, 
small and large, in all their varieties. The members of the erythrocyte series seem 
greatly to outnumber those of the leucocyte series in the spleen. 
(17) The leucocytes are in great abundance before the thymus rudiment appears, 
and at the end of the larval series examined, the gland is still a mass of epithelial cells, 
showing no resemblance to. the lymphoid cells in other tissues. There are practically no 
leucocytes in its substance, and no special grouping of them, either in the surrounding 
mesenchyme or in the veins. 
SUMMARY OF INTERPRETATION. 
The study of the early corpuscles shows that they are at first all alike—probably 
wandering cells, such as those first described in the living Teleost embryo by WrENcKE- 
BACH.~ Almost immediately a change sets in, which leads to the adoption by most of 
them of a passive role, while others remain free mobile elements, with centrosome and 
sphere. 
The latter have all the morphological characters of leucocytes, and exactly similar 
cells occur in the somatic mesenchyme. As this is long before hemoglobin is developed, 
the cells are never at any stage wholly heemoglobin-containing corpuscles; and if these 
leucocyte-like cells are the successors of the early corpuscles, mobile elements are never 
absent from the blood. It is possible that we should call the early corpuscles them- 
selves ‘leucocytes.’ This may seem an unwarrantable use of the term, but I believe it 
might bear examination. ‘The leucocyte is generally admitted to be the phylogenetically 
older cell. In the Dipnoi the blood is extraordinarily rich in leucocytes, and they 
appear at a very early stage. The postponement of the appearance of the leucocytes in 
Ontogeny, supposed to exist in all vertebrates, has, it might be considered, only 
* Journal of Anat, and Phys., vol, xix., 1885 ; Archiv f. Mikr. Anat., Bd. 28, 1886, 
