228 PROFESSOR FRANK J. COLE 
the Myxinoid are also formed by the coalescence of a vascular network. On this view, 
the vascular papille will represent the remains of the embryonic plexus. In the closed 
papille the detachment of their contributions to the sinus will have been complete, 
whilst the open papillze will have retained the embryonic continuity of the two systems 
of spaces. 
As regards Petromyzon there are some grounds for this view. Cori points out 
that the branchial sinuses are represented in earlier stages by what he calls a venous 
network. ScHNEIDER also has the following significant passage: ‘‘ Zwischen der 
Kpithelialschicht der Mundhéhle und der Muskulatur de Korperwand befindet sich im 
Ammocétes eine Schicht sogennanten adenoiden (cytoiden Binde-) Gewebes, welches 
von zahlreichen Capillaren durchsetzt wird. Beim Uebergang in den Petromyzon 
nehmen diese Capillaren an Grésse zu und verschmelzen zu einem grossen Venensinus.” 
The fact that all the so-called lymph spaces in Myxinoids normally contain blood, 
though in varving quantities, must be held to remove these spaces from the category 
of lymph spaces sensu stricto. At the same time, however, they cannot be said to lie 
in the direct course of the blood stream, and for this reason must be equally excluded 
from the blood vascular system. LancEruans declines to commit himself as to what 
system the spaces belong to, but later writers have generally referred them to the 
lymphatic system, and denied that blood is normally present in them. Mozxsxo, 
however, takes the intermediate course, and regards the spaces as blood-vessels and 
lymphatics at the same time. 
There is, to my mind, little doubt that as regards the vascular system the Marsipo- 
branch fishes have reached the parting of the ways. The blood system is well developed, 
but not completely developed. There is still im places an ill-defined connection between 
the arteries and the veins. Similarly, the lymphatic system is indicated as far as its 
broad outlines are concerned, but it contains red blood, and has not yet acquired its 
independence. It is, in fact, in the act of becoming detached, as is illustrated by the 
great variation in the amount of blood it contains, and in the fluctuating extent to 
which it receives blood from the arteries via the vestigial vascular papillee. 
SUMMARY. 
1. Red blood occurs normally, but to a variable extent, in all the so-called 
“lymphatic” spaces of Myaxine. In the living animal such blood can readily be seen 
in the extensive sub-dermal sinus. 
2. In the region of the gills, and especially in the case of the peribranchial or pleural 
sacs, red blood enters the spaces from the arteries. This has been actually observed 
under the microscope in certain injection experiments on the freshly killed animal. 
The means of communication between the arteries and the sacs are a number of © 
perforated papill situated on the afferent and efferent branchial vessels. The blood 
so entering the sacs is drawn off again into the venous system via the jugular veins. 
