42 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Klaatsch has been working towards a reconstruction of the original ver- 
tebral column. 
The same author * draws a sharp distinction between the perichordal 
and the chordal origin of vertebrae. In the perichordal mode, described by 
Gegenbaur, and Balfour and Parker for Lepidosteus, a continuous cartila- 
ginous tube surrounding the notochord becomes by segmentation divided 
into vertebrae. The elastica and the non-cellular sheath play no part. 
In the chordal mode, which the author describes in detail, relations are 
established between the cartilaginous arches and the elastica ; as a closed 
membrane the latter disappears ; the notochord-sheath is opened up to, 
and invaded by the cartilage ; there is a competition of elements, and 
the cartilage gains the day. Along these two lines of perichordal and 
chordal evolution, the vertebral column of fishes has evolved. On the 
one (perichordal) line we have the Cyclostoma, Teleostei, Lepidosteus, 
and Acipenser ; on the other (chordal) line, we have Dipnoi, Holo- 
cephali, and Elasmobranchs. 
Origin of Vascular System in Selachii.| — Prof. C. K. Hoffmann 
has investigated this in embryos of Acanthias vulgaris, and finds that the 
endothelium of the entire vascular system arises from hypoblast. In 
fact, heart, aorta, omphalo-mesenteric veins, truncus arteriosus, &c., are 
constricted off from the archenteron. The development of the heart 
and aorta is introduced by a remarkable “ struggle for life ” among the 
hypoblast cells of the archenteric wall ; and in this struggle a multi- 
tude of cells perish. Many of the survivors, destined to become endo- 
thelial, exhibit a modification into long spindle-shaped elements ; others 
have numerous exceedingly fine processes and often form chains or a 
mesh work. This peculiarly modified archenteric epithelium the 
author calls heemenchyme-tissue ; most of it becomes endothelium, while 
some elements seem to become blood-cells. But the most important 
result is the general conclusion stated above that the endothelium of the 
vascular system is entirely hypoblastic. 
Prof. C. K. Hoffmann continues J his investigation of the development 
of the venous system in Selachians. It is likely that ancestral forms 
had two omphalo-mesenteric veins, but in extant forms the right vein is 
represented merely by two rudiments, an anterior portion opening into 
the sinus venosus, and a smaller posterior portion which unites with the 
left vein. The posterior part soon aborts ; the persistent anterior por- 
tion subsequently unites dorsally with the left vein directly behind the 
pancreas. A second ventral anastomosis forms a venous ring around 
the gut. Later on the mesenteric vein (the sub-intestinal of the trunk) 
unites with the left omphalo-mesenteric vein and forms a second 
venous ring. Thus the Selachians are shown to agree in an important 
respect with the Amniota, in all of which there are two such venous 
rings. 
In the sub -intestinal vein, two portions must be distinguished, the 
caudal and the trunk portion. The former loses connection with the 
latter, and unites with the cardinal veins. But there is a third portion, 
* Morphol. Jahrb., xx. (1893) pp. 143-86 (1 pi., 6 figs.). 
t Op. cit., xix. (1893) pp. 592-648 (4 pis., 6 figs.). 
x Op. cit., XX. (1893) pp. 289-304 (1 pi.). 
