133 
with the cavity of the coenosarc, from which it thus forms a simple 
diverticulum. We next find that the bud has become differentiated 
into a peripheral and a central portion, the latter containing the 
diverticulum from the cavity of the coenosarc. A further differen- 
tiation is soon seen in the central portion, which is now manifestly 
composed of two layers, — an ectoderm and an endoderm, — while the 
peripheral portion becomes enveloped externally by a delicate chiti- 
nous investment. This peripheral portion is to become the ectotheque 
of the simple gonophore. 
From the summit of the central portion, which will afterwards 
become elongated into the manubrium of tlfe medusa, four thick 
cylindrical coeca may be now seen to be given off ; their cavities are 
simple continuations of the diverticulum, and they extend, with their 
sides in close contact with one another, towards the distal extre- 
mity of the gonophore. They are at first mere tubercles, but they 
gradually increase in length, becoming developed quite in the same 
way as the tentacles on the ordinary alimentary polype. After 
they have attained a certain length, the gonophore continuing at 
the same time to grow larger, they became separated from one an- 
other laterally, and the intervening spaces are now seen to be 
occupied by a web-like membrane, which extends from the base of 
the coeca to within a short distance of their extremities, and here 
terminates by a defined margin. 
This membrane is composed of transversely elongated cells, and is 
plainly a lateral extension of the ectoderm of the coeca. Close to 
its free margin a narrow tube may be seen extending transversely 
between the four coeca, connecting them together, and becoming more 
and more elongated as the coeca continue to separate from one 
another. 
In the four coeca it is now easy to recognise the radiating canals 
of the gastrovascular system of the developing medusa, and in the 
transverse tube, the circular canal of this system, while the connect- 
ing membrane is plainly the rudimental umbrella. 
The distal extremities of the coecal tubes project slightly beyond 
the margin of the umbrella, and here become dilated into bulbous 
terminations, which converge towards the axis of the gonophore, 
where they lie in contact with one another, and soon become very 
conspicuous by the accumulation in them of red pigment. From 
each bulb two short tentacles may now be seen to sprout, and as 
VOL. iy. 
