ON THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES. 227 
4. A papilla may be completely injected, so that the capillaries within the contiguous 
walls of the artery are filled, without, however, any signs of the injection escaping from 
the papilla. In these cases the interior of the papilla is completely occupied by the 
blood space, and there is no distal spongy end. 
5. In one injection the free spongy extremity of the papilla was traversed by several 
of the blood vascular tubules, some of which were observed to unite to form a large 
vessel opening on to the exterior by a small pore. 
6. In another injection the stalk of the papilla contained the customary large 
simple blood cavity, from which there were given off distally a number of tubules, the 
latter combining in the distal bulbous end of the papilla to form an elaborate vascular 
network. 
7. From the large central blood space of the papilla there may pass into the more or 
less solid tissue of the free end a number of intercellular blood canalicult. These differ 
markedly from the much larger and well-defined blood vascular tubules, which may also 
be present. ‘They cannot be described as blood capillaries, since they have no definite 
walls, but rather resemble the bile capillary. 
4, THEORETICAL CONSIDERATIONS. 
The development of the lymphatic system has of late years been successfully 
studied by several American morphologists. The most recent paper on the subject 
is by Huntington on the development of the lymphatic system in Reptiles.* Ac- 
cording to this author, the systemic lymphatic channels arise by the confluence 
of mesenchymal spaces independently of the blood vascular system. They are 
not derived from veins, their cavities are independent of those of the veins, and they 
are lined with a “‘ lymphatic vascular endothelium not derived from or connected with 
a pre-existing hemal vascular endothelium.” 
On the other hand, the jugular lymph sac, both of Reptiles and Mammals, has an 
altogether different origin. This arises by the fusion of a venous plexus, directly 
derived from the venous channels of the pericardial area. ‘The endothelium of the veins, 
_ therefore, is continuous with that of the lymph sinus, and the latter at first contains 
red blood. ‘It then evacuates its early blood contents, separates temporarily from the 
heemal vascular system, establishes secondary connections with the systemic lymphatic 
vessels of the anterior part of the body and of the anterior extremity, and finally re- 
enters the venous system at a definite and constant point of secondary lymphatico- 
venous junction.” 
Weare unfortunately ignorant of the development of the peribranchial sacs and other 
so-called lymph spaces in the Myxinoid fishes, but the facts disclosed by the American 
morphologists suggest a possible explanation, or at all events indicate a lme of inquiry. 
If the jugular lymph sac of Reptiles and Mammals is formed by the fusing up of a 
venous network, and a sinus thereby formed, it may be that the peribranchial sacs of 
* Anat. Rec., June, 1911. Cp. also January, 1910, and Mem. Wistar Inst. Anat. Biol., No, I., 1911. 
