MUSEUM OF COMPARATIVE ZOOLOGY. 129 



PI. V. fig. 2, vp, one * of these " vasn-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 Echinarachnius, 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 oeso- 

 phagus, stomach, and intestine, PI. 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, PI. IV. fig. 3, sp. 



The calcareous rods appear on each side of one of the openings into 

 the gastrula stomach. In Echinus, according to Krohn, the primitive 

 invagination, or the blastopore, becomes the vent of the pluteus. A. 

 Agassiz says the same of the gastrula of Strong}'locentrotus. 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 amoe- 

 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, PI. IV. fig. 3. As in Oj)luo- 

 pholis, these structures arise in the bilateral masses of mesoblastic 

 cells,t 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 amoeboid 

 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 

 difficvdt to observe the formation of the external opening. In one 

 specimen, PI. 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 sincrle vaso-peritoneal sac constricts from the stomach 

 of tlie gastrula. This sac later divides into the right and left vesieles. 



t A. Agassiz has already called attention to the fact tliat the limestone rods are 

 first deposited in the midst of similar cells, to which he gives the name "yolk 

 cells." Revision of the Ecliini, p. 712. 



VOL. XII. — NO. 4. 9 



