30 
MONICA TAYLOR. 
Reference was made in the introdiicfcion to a ^‘pouch^^ or 
pocket given off from the roof of the mouth. Traces only 
of this pouch can be found in the young creature, but 
sections through the adult show that it is an invagination of 
the lining of the gill-cavity, probably a device for increasing 
the area of the respiratory surface. This invagination begins 
immediately behind the attachment of the first branchial arch 
to the pharynx, and the pocket so formed extends backwards 
with a gradually decreasing diameter to the attachment of 
the fourth arch. The opening into the pocket is large, longer 
than broad. 
(b) The Excretory System. 
Solid pronephric nephrotomes are discernible about Stage 
18. 
There is little to distinguish the rudiment of the pro- 
nephros from that of its duct. The nephrotome of the third 
trunk myotome appears to give rise to the functional pro- 
nephros, those of succeeding segments forming the archi- 
nephric duct. It is not possible to say whether other segments 
besides the third have any share in the formation of the pro- 
nephros, but in the light of subsequent events it does not 
seem likely. 
The separation, which begins at Stage 20, of the various 
nephrotomes from their respective segments to become con- 
verted by a process of fusion into an archinephric duct seems 
to take place practically simultaneously — the hinder parts of 
the duct being formed as soon as the anterior portions. 
Stage 21. — The third nephrotome has become more deep- 
seated. Although still solid it has increased in size because 
of the development of lacun8e, these lacunae being the first 
indication of the pronephric chamber. The archinephric 
duct rudiment is still solid, round and cord-like, lying approxi- 
mately on a level with the dorsal aorta. 
Stage 24. — The rounded pronephric chamber rudiment is 
now being converted into a typical Malpighian capsule, and 
the formation of a glomerulus from the dorsal aorta has 
