THE HISTOLOGY OF THE BLOOD OF LARVA OF LEPIDOSIREN PARADOXA. 455 
nuclei, so that they take a deeper coloration. The shape of the nucleus varies, but 
is most frequently horseshoe-shaped, and the protoplasm is either hyaline or granular 
in different degrees. Though it is true that at the earlier stages leucocytes are 
wandering in from without, I think there is little doubt that these cells are being 
formed at the expense of the cells of the columns or trabecule, which, after all, have 
exactly the same origin as the remainder of the mesenchyme, in which we have seen 
the cells being transformed into similar elements. 
The mononuclear cells have certainly a local origin. They are seen crowding the 
splenic vein (fig. 27, Pl. III.). They are of varying dimensions, and are indistinguish- 
able from the cells with simple nuclei first observed in the blood at stages 31 and 32. 
The problems regarding them are exactly the same as presented themselves at the 
earlier stages, but I shall postpone discussion of the point until I consider the next 
stage, as I have had the opportunity of staining a series of sections freshly cut from 
that stage with stains specially suited for discriminating between the different types 
of cells. 
At stage 37 the spleen is an elongated organ, overlapping the pancreas behind and 
extending forwards along the whole length of the liver to a level close behind the 
point where the lung comes off from the pharynx. It is broad behind, but tapers in 
front, and it is in this anterior part that the general structure can be most easily 
determined (fig. 32, Pl. IV.). 
Surrounding the central artery there is an axial mass of cells with small irregular 
nuclei. From this to the periphery extend radiating trabecule, with small oval or 
elongated nuclei. Between the trabeculz are large sinuses packed with cells belonging 
to different categories, and in the meshes of the framework immediately round the 
central area are large and round nuclei, which sometimes, it is quite clear, have a zone 
of free protoplasm round them, while in other cases they seem imbedded in the general 
protoplasmic framework. In the apex of the organ the structure is simple because this 
zone is much reduced, but posteriorly it is more extensive and more loosely arranged, 
so that the picture is more complicated. It is a matter of great difticulty to say some- 
times whether the large round nuclei with which this zone is studded belong to the 
trabecular framework or to cells in its meshes. 
In the peripheral sinus, and the larger sinuses between the outer ends of the 
trabecule which are continuous with it, there are great numbers of hemoglobin- 
containing erythroblasts. These and the erythrocytes give to the outer zone of the 
spleen characters which would justify the application to it of the name of ‘ pulpe rouge’ 
used by Lacunssz,* while the central portion is the ‘ pulpe blanche.’ 
The free cells in the spleen pulp belong to several different categories, and the 
discrimination between them is a matter of difficulty, as all the nuclei stain blue with 
hematoxylin, and the only differences are those of intensity of staining, associated 
with a difference in the disposition of the chromatin. I was unable, except in a 
* Jour. de? Anatomie et de la Physiologie, T. xxvi., 1890. 
