458 Mr. C. Grave on the 



found of its originating as two bands, and is continued 

 until the segmentation-cavity is quite crowded with cells. 

 Figure 2 is a camera sketch of a section of a larva at the 

 time of hatching, cutting it in the plane of the long axis, 

 showing the shape of the larva at this stage and the method 

 of mesenchyme formation. 



The cilia did not show in the section, but those at the 

 animal pole are much longer than those over the rest of the 

 larva. 



The gastrula-stage is formed several hours later by the 

 invagination of the vegetable pole. The cells composing the 

 invaginated tube or archenteron are all of about the same 

 shape and size, but a decided differentiation is to be noted 

 among the ectoderm cells. Those at the animal pole are 

 much elongated and vacuolated, thus forming a thickened 

 apical plate. The lateral walls, too, have each a thickened 

 area, while the cells of the ventral side are of a uniform 

 thickness, but much thicker than those composing the dorsal 

 surface of the larva, which are much flattened and thin. 

 Near the thickened lateral areas clumps of mesenchyme cells 

 collect and begin to secrete the larval skeleton. Beside 

 these, other mesenchyme cells take up the function probably 

 of support for the archenteron and other organs as they form. 

 At least in gastrulse and older larvae long branching cells can 

 be seen in the segmentation-cavity connecting the archen- 

 teron with the ectoderm wall or with other mesenchyme cells, 

 or connecting two portions of the ectoderm. The cells of the 

 supported parts to which the processes of the mesenchyme 

 cells attach take an active part in the formation of the 

 connecting fibre or strand, as part of their substance meets 

 and fuses with that sent out by the mesenchyme cell. 



From the apex of the flattened archenteron a pair of 

 pouches grow out and are constricted off, one to the right, the 

 other to the left. Each of these divides into two pouches of 

 about equal size, one of which remains in about its original 

 position, while the other migrates toward the blastophore and 

 takes up a position on the side of that part of the archenteron 

 which will later become the stomach. This is as Metschni- 

 koff described it and as Bury supposed must be the case; but, 

 contrary to what both the above investigators describe, I find 

 that the right posterior pouch degenerates and disappears, 

 thus leaving a larva with a pouch on either side of the 

 oesophagus and one on the left side of the stomach. This 

 condition was noted in every pluteus observed, and hundreds 

 of them have been studied. 



In confirmation of the phenomenon observed in Ophiurans 



