I GROWTH OF THE GERM-CELLS 5 



ventral side, consists of a single cell-row, the other, dorsal, of 

 two or more rows. The embryo is now visibly bilateral. 



By the immigration of the primary mesenchyme the pig- 

 mented cells are naturally brought down to the vegetative 

 pole, and they are now invaginated into the interior. The 

 inner tube so formed is the archenteron, the aperture of 

 invagination the blastopore. The archenteron contains the 

 material for the body cavity and water- vascular system and 

 the alimentary canal, the blastopore persists as the anus. 

 Cells are budded off from the inner end of the archenteron to 

 form the secondary mesenchyme, and then this inner end 

 enlarges, divides into two, and these are nipped off as the 

 right and left coelom-sacs. Each of these later divides into 

 three pre-oral coelom, hydrocoel, and posterior coelom ; but 

 that is a process which we cannot follow here. 



The rest of the archenteron is now the gut. It quickly 

 becomes divided into the fore-, mid-, and hind-guts. It is 

 inclined towards one, the ventral, side, where the fore-gut 

 soon unites with an ectodermal depression, the stomodaeum. 

 By perforation at the point of union the mouth is formed. 



Meanwhile the ventral or oral side has become flattened 

 and square, the opposite dorsal side convex, and the ' prism ' 

 form is attained. A sense-organ a tuft of long cilia borne 

 by a patch of thickened ectoderm is developed at the 

 anterior end. 



The edges of the square oral side now become thickened, 

 while their cells put out long cilia, so producing the circum- 

 oral ciliated ring, by means of which the larva swims. The 

 four corners of the ring are then pushed out into the four 

 primary arms of the Pluteus, two anterior or antero-lateral, 

 two posterior or postero-lateral, and each is supported by 

 a rod of the skeleton. Of the three radii of the primary 

 triradiate spicule, one grows into the postero-lateral arm on 

 each side, one forms a horizontal bar reaching to the middle 

 line below the gut, and the bar supporting the antero-lateral 

 arm is a branch of this. The third radius passes dorsally to 

 the convex side, and becomes the apical or body rod. Its 

 extremity is swollen and club-shaped. The arms and their 



