464 |. DR THOMAS H. BRYCE ON 
partially taken place in Lepidosiren. PARKER™ says that in Protopterus the leucocytes 
bear a larger proportion to the red cells than in any other vertebrate, except in patho- 
logical conditions. It is a possibility that in Lepidosvren we have a stage in which the 
blood vascular and the lymph vascular systems are not sharply marked off from one 
another. Perhaps we see in the blood the division of labour going on, which makes one 
primitive corpuscle a respiratory, another a lymph cell. The phenomena of phase 2 
support such a supposition. 
Up to stage 30, when the hemoglobin appears and phase 2 sets in, all the cor- 
puscles, active or passive, have nuclei of the leucocyte type; the chromatin is collected 
into karyosomes, rather widely scattered, with a fine filamentous intervening reticulum. 
The erythroblasts in the following stages have the erythrocyte type of nucleus, richer in 
chromatin, with large closely-set chromatin nucleoli, jomed by a coarser and more dis- 
tinetly reticular intervening substance. (Compare figs. 13, 14, 15, 19, Pl 1.) At 
stage 31, and better marked at stage 32, the place of the earlier cells with nuclei of the 
leucocyte type is taken by the large mononuclears. These have either a single or a— 
double origin. It may be that they are all derived from the splanchnic mesenchyme, 
and that they wander from thence, especially into the nephric tract of the somatic mesen- 
chyme., The distribution of cells in the cardinal veins is, however, against this, and I 
am disposed to believe that they arise in both situations. The point is not very 
material—they are mesenchymatous in origin in either case.t 
Exactly similar cells to those mononuclears in the blood stream become outside the 
blood stream leucocytes of the several varieties. From the larger number of those 
mononuclears in the blood, compared to the number of polymorphonuclear cells, while 
the opposite proportion prevails outside the blood stream, and from their identical 
characters with the basophile cells of earlier stages, I believe it is a logical induction, 
not an assumption, that they become converted in the blood into the heemoglobin- 
containing elements. Mitoses are frequent in those cells outside the blood stream, but 
they do not occur in the blood stream itself. This inference is confirmed by the observa- 
tions on the spleen; and thus I conclude that from common mother cells added to the 
blood during phase 2 from the mesenchyme, and in phase 38 especially from the spleen 
(a derivative of the mesenchyme), are derived two families of cells, one undergoing their 
metamorphosis in the blood to form the respiratory erythrocytes, the other undergoing 
their metamorphosis outside the blood stream in the connective tissue, v.e. lymph spaces, 
to form the typical wandering polymorphonuclear leucocytes. } 
* Trans. Roy. Irish Academy, vol. xxx. part iii. p. 168. 
+ I am here assuming that the facts justify me in concluding for a continuous new formation, at least up to the 
end of phase 2, The statement in the text is perhaps too sweeping. If it were possible to establish that all the 
colourless cells were derived from the splanchnic mesenchyme, and that they wandered thence, it would be a fact of 
great significance, as it is derived directly from the primitive hypoblast, and it might be held that the free cells im it 
were directly derived from that layer. I think this would be a strained interpretation, and difficult to reconcile 
ee described regarding the nephric tract of the somatic mesenchyme, but it is a possibility which must not 
{An objection might here be raised that the nuclei of the polymorphic cells may regain their simple form in the 
blood, but as there are many polymorphs in the blood this objection would not have much force. 
