THE DEVELOPMENT OP SYMBIIANC HUS MARMORATUS. 33 
more epithelial character. The slightly club-shaped end 
of each tubule is hollowed out into a rounded cavity very 
like that seen in the development of the pronephros, and a 
mass of darkly stained mesenchyme in the neighbourhood of 
each of these Malpighian capsule rudiments, probably repre- 
sents the rudiment of the glomerulus. 
Stage 33. — Many mesonephric tubules have acquired an 
opening into the archinephric duct which is distinguishable 
from the former by its paler staining powers. The Mal- 
pighian capsules in many of the segments are fully formed — 
the glomerular artery arising from the dorsal aorta by a 
common root with an intersegmental artery (PI. 4, fig. 27). 
Stage 34. — Horizontal sections at this stage show the 
segmental character of the mesonephros at its best. By this 
time all the mesonephric rudiments have fully developed and 
Malpighian capsules with their glomeruli and tubules occur, 
one in each segment. The urinary bladder has lengthened 
out and the pseudo-lymphatic tissue has increased. 
Stage 35. — Although externally the creature is quite 
adult in appearance the mesonephric tubules have not yet 
attained their complete development. They are still readily 
distinguishable from the duct by their staining reaction, the 
cells of the duct being much redder in preparations stained 
with eosin and haemotoxylin than the purple ones of the 
tubules. 
The very much coiled anterior end of the duct is embedded 
in a capsular structure formed of pseudo-lymphatic tissue, 
the elements of which are smaller and more densely packed 
than in the earlier stages. Posteriorly the tissue has to make 
way for the tubules, and is not so abundant. 
Stage 36. — The epithelial lining of the tubules has now 
quite an adult appearance. New secondary tubules are 
beginning to form between the earlier tubules. These new 
rudiments repeat the developmental history of the first formed 
units, being closely packed, densely stained tissue masses in 
the immediate neighbourhood of the duct when they first 
appear. Eventually they become coiled tubules. However, 
VOL. 59, PART 1. NEW SERIES. 3 
