THE HISTOLOGY OF THE BLOOD OF LARVA OF ZEPIDOSIREN PARADOXA, 449 
of possible significance, that this specialised tract corresponds in situation to the original 
seat of origin of the ‘sclerotome’ from the mesoblast;* while the splanchnic mesen- 
chyme is the differentiated representative of a previously undifferentiated mass of 
primitive hypoblast cells which had certainly an important share in the contribution 
of the early blood corpuscles. 
I must also state that I have searched more than once every section through the 
pharynx and gill clefts at this stage for thymus ‘placodes’ giving origin to leucocytes, 
as described by Brarp.+ I cannot identify any thymus rudiment, nor have I seen 
any appearances like those figured by Brarp. Moreover, though leucocytes occur here 
JO 
5 oO 
30 
Fie. 4.—Section through larva, stage 32+, behind liver. x88 d. 
A, aorta; C.V., cardinal vein ; N, specialised tract of mesenchyme in region of mesor ephros, 
and there in the general mesenchyme in that region, and also in the aortic arches, I do 
not happen to have seen a single one in the anterior cardinal vein. 
Splanchnic Mesenchyme. 
It has already been observed that this tissue surrounding the isolated portion of 
the gut is composed of rounded closely packed cells. There are numerous spaces in it 
containing red blood corpuscles, and here and there free elements with characters 
similar to those of the cells immediately to be described. 
Dorsal to the gut, a tract is showing the first stages of differentiation from behind 
forwards, which will convert it into the spleen. The layer on the yolk which is in 
direct continuity with the undifferentiated hypoblast has, however, very special 
characters. Ifa section be taken at Y in text fig. 1 it will be seen that the tissue is 
composed of loosely arranged fusiform or stellate cells, with many free elements in the 
* I find a remark almost in the same terms as this was made by ZEIGLER (Arch, mikr. Anat., Xxx., 1887), 
referring to the similar tract in Teleosts. See also Lacussss, loc. cit., p. 364. 
+ Loc. cit. 
