ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
571 
organ, and briefly notes his own results in the case of birds. He finds 
that in the fifth day of incubation in duck erabrvos the primordium of 
the spleen can be seen as a cluster of mesoderm cells which lie in the 
left dorsal region of the mesentery, near the dorsal pancreas primordium. 
The cells have no connection with those of the future pancreas. The 
coelomic epithelium is thickened in the neighbourhood of the splenic 
and pancreatic primordia, but takes no part in the formation of either 
organ. 
Development of the Trout/* — MM. A. Swaen and A. Bracket have 
made a special study of the development of the organs derived from the 
mesoblast in the trout. The points to which they have directed their 
attention are, the segmentation of the mesoblast, the development of the 
vascular system and of the sclerotomes, and the origin and morphological 
significance of the pronephros. As to the pronephros, they find that, in 
its first beginnings, this organ arises by an isolation of the internal part 
of the splanchno3cele from the fourth somite to the cloaca. The cavity 
thus shut off becomes in the posterior region the pronephric duct, but 
more anteriorly forms the pronephric chamber. The pronephric duct 
is, therefore, a rudimentary pronephric chamber. The chief difference 
between the true pronephric chamber of the trout and that of Ichthy - 
ophis is the absence in the former of the Aussentrichter of the latter. 
According to the authors the pronephric chamber of Teleosteans is the 
homologue of the dorsal part of the coelom in Petromyzon and Amphi- 
bians, and the degree of isolation of the cavity varies in the different 
Vertebrates according to the functional importance of the organ. In 
Teleosteans the pronephric duct exhibits in rudiment the parts of the 
pronephros, showing that the organ once extended throughout the greater 
part of the body. As to the bearing of these results upon Riickert’s 
views as to the phylogeny of the pronephros, the authors believe that it 
is necessary to regard the pronephric chamber as an integral part of the 
primitive pronephros, this pronephric chamber being defined as that 
portion of the splanchnocoele into which the tubules open, aud whose 
walls contain networks of blood-vessels derived from the aorta. 
As to the other mesoblastic structures, the authors find that the 
elements from which the vascular system originates are distinct from 
the mesenchyme which gives rise to the connective-tissues and probably 
to the lymphatic system. The vascular system developes in two distinct 
areas of the embryo, the cardiac endothelium and the migratory vascular 
cells arising anteriorly, and the aorta, median vein, and formed elements 
of the blood posteriorly in the segmented region of the embryo. They 
do not believe that the cavities of the blood-vessels are coelomic, but 
emphasise the distinctness in origin of lymphatic and blood- vascular 
systems. The blood-system is certainly mesoblastic in origin in Teleos- 
teans, but they are unable to explain the reason of the distinction from 
Amphibians, where, according to several authors, the blood-system origi- 
nates in the hypoblast. 
Origin of Paired Fins in Selachians. - ) - — Dr. Hermann Braus, as a 
part of his researches on the development of the musculature and peri- 
* Arch, de Biol., xvi. (1899) pp. 173-311 (7 pis.). 
f Morphol. Jahrb., xxvii. (1899) pp. 501-629 (4 pis. and 6 figs.). 
