DEVELOPMENT OF CUCUMARIA ECHINATA 201 
of the same vesicle, on the contrary, moves to the left through 
the dorsal side (fig. 16 c, en), and, as the two parts thus move 
in opposite directions, they gradually begin to be cut off from 
each other a little on the left side (Pl. 8, fig. 15). 
The anterior part, which will give rise to hydrocoele (hy), 
gives out from the postero-dorsal margin obliquely backwards 
a conical process which finally unites with the dorsal ectoderm 
(pe). This is the rudiment of the pore-canal. The posterior 
part, which is the future enterocoele (en), is a little smaller 
than the anterior part and lies on the left side, extending round 
the body-axis and stretching from the antero-ventral to the 
postero-dorsal side. 
The walls of the hydrocoele and enterocoele consist of a single 
layer of cells, clearly distinct from the free mesenchyme cells, 
and the latter do not yet attach themselves to the surface of 
the former. 
The first observer who traced the fate of the archenteron was 
Selenka (45, p. 170). He noticed that in C. planci the 
archenteron bifurcated at the top, and that the dorsal branch 
increased rapidly in size, bending obliquely antero-ventrad, 
and at last becomes separated as a vaso-peritoneal vesicle from 
the other branch, which latter was stunted and later gave rise 
to the gut (‘ Kérperdarm’). In my opinion, his two branches 
are a complete vaso-peritoneal vesicle, and he seems to have 
overlooked the separation of the gut from that vesicle. He 
further stated that after the separation of the two vesicles the 
vaso-peritoneal vesicle shifted to the left side of the gut, while 
the latter rapidly grew forwards and at last united with a ventral 
invagination (“Munddarm’). He is right in saying that the 
first part then hes on the left side, but that the gut breaks 
through to the stomodaeum is improbable at such an early 
stage, and moreover, the part which he called ‘ Kérperdarm ’ 
is, | think, to be identified with the enterocoele, which should 
never have any communication with the stomodaeum at all. 
A careful comparison of his figs. 21, Pl. xi, and 22.8, Pl. xii, 
leads one to conclude that the part he denoted P (enterocoele) 
in the fig. 22 B is derived from that part denoted B in the fig. 21, 
