Metamorphosis of Axolotls 
53 
gills grafted on to the larvae of salamanders, but he does not 
describe histological changes which occur during metamorphosis. 
The gill of an axolotl is composed of three strong trunks whose 
diameter at the root attains about 5 mm., the length 18 to 20 mm. 
On their lower and their side surfaces they are covered with 
numerous filaments. Under the magnifying glass we observe blood¬ 
vessels: the branchial veins, situated on the dorsal part, through 
which blood flows in the direction of the animal’s bod} 7 , and the 
branchial arteries lying ön the ventral part which carry blood 
from the heart towards the gills. In microscopical dissections, we 
distinguish here the many-layered flat epithelium with numerous 
cells of Leydig, much loose connective tissue containing blood¬ 
vessels, pigment and strings of striated muscles. Gill filaments have 
the form of narrow leaves; their length attains 6 to 8 mm.; they 
are sometimes bifurcated. In a microscopical dissection of the 
gill-filament of a black axolotl (fig. 5) we see the two-layered flat 
epithelium composed of an external layer of high cells having 
globular nuclei, and of an internal layer of low cells with long 
flat nuclei. 
Faussek says that the gill filaments of an axolotl of 6 cm- 
length are covered with one layer of epithelium; in my specimens, 
which attained 20 cm.. I have found two epithelial layers in 
microscopical dissections of gill-filaments. Only one layer was 
sometimes to be seen in the neighbourhood of large vessels. The 
blood-vessels on the narrow verge of the filament (the branches 
of the branchial vein and of the branchial artery) are considerablv 
developed in comparison with the size of the filament; their diameter 
was about 190 m. They contain red and white blood-cells. Round 
the vessels pigment gathers which is also observable between the 
small capillary vessels in the middle of the filament. We notice 
scanty quantities of connective tissue, consisting of thin fibres and 
triangular nuclei. The figure in Faussek’s paper probably refers 
to younger specimens; it shows more connective tissue and the 
blood-vessels lie only on the margins of the filament, directly under 
the epithelium. 
The degeneration of gills sets in at a time when no loss of 
weight of the animal was yet observed. A few days after the 
beginning of thyroidine-feeding, the gill-trunks become already 
somewhat shorter and the filaments narrower; they lose the form 
