MUSEUM OF COMPARATIVE ZOOLOGY. 129 
Pl. V. fig. 2, up, one * of these ‘‘ vaso-peritoneal vesicles,” now con- 
stricted from the enteron, is seen as a closed sac on one side of the 
gastrula stomach. The beginnings of the formation of pouches, which 
probably form the water-tubes, were observed in Kchinarachnius, but 
I have not traced them in their later stages of growth. I have ob- 
served only one of these constricted off from the gastrula stomach. 
The vesicle is separated from the enteron before the ‘“‘mouth opening” 
is formed. The wall of the infolded pouch now begins to differentiate 
itself by constrictions into three regions, corresponding with the ceso- 
phagus, stomach, and intestine, Pl. IV. fig. 5, of the fully grown plu- 
teus. At about the same time, also, the limestone rods or calcareous 
framework of the pluteus first appears, Pl. IV. fig. 3, sp. 
The calcareous rods appear on each side of one of the openings into 
the gastrula stomach. In Kchinus, according to Krohn, the primitive 
invagination, or the blastopore, becomes the vent of the pluteus. A. 
Agassiz says the same of the gastrula of Strongylocentrotus. I have no 
observation on this point in the gastrula of Echinarachnius, and noth- 
ing to show that there is any difference in this genus from what is 
recorded in Strongylocentrotus and other Echinoids. 
The formation of the two limestone spicules which characterize the 
pluteus at this age, takes place in the cluster of mesoblastic or amce- 
boid cells, acl, on each side of the opening, which henceforth serves as 
the mouth of the pluteus. The first appearance of the limestone rods 
was detected in a gastrula one day old, Pl, IV. fig. 3. As in Ophio- 
pholis, these structures arise in the bilateral masses of mesoblastic 
cells,f one on each side of the blastopore or oral end of the stomach. 
They are at first disconnected, branched, or stellate, and trifid, resem- 
bling small sponge spicules. Around them are clusters of the ameboid 
cells, from which they form.f The neighboring epiblastic wall of the 
gastrula is reddish and yellow. It was also noticed that at the lowest 
point of the infolded pouch the same color is prominent. At the last 
mentioned position the aggregation of cells and pigment renders it very 
difficult to observe the formation of the external opening. In one 
specimen, Pl. V. fig. 2, cl, I noticed an infolding of the ventral wall 
opposite the lower end of the invaginated stomach of the gastrula, 
* According to Selenka, a single vaso-peritoneal sac constricts from the stomach 
of the gastrula. This sac later divides into the right and left vesicles. 
t A. Agassiz has already called attention to the fact that the limestone rods are 
first depesited in the midst of similar cells, to which he gives the name “yolk 
cells.” Revision of the Echini, p. 712. 
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