2l8 THE ASCIDIANS. 



vesicles.* All these structures are differentiated from the 

 loose mesenchyme cells, all of which at first course round 

 about the body of the young Ascidian like blood, being 

 kept in motion by the beating of the heart. 



In the stage shown in Fig. 105 A the mesodermic bands 

 are still fairly compact in front, having extended them- 

 selves anteriorly at the sides of the enteron by interstitial 

 growth. 



PrtEoral Body-cavity and Praoral Lobe. 



When the larva first hatches, the endoderm and ecto- 

 derm are in contact with one another at the anterior 

 extremity of the body, just as they are in the earlier 

 stages. (Cf. Fig. 102.) Soon, however, the ectoderm, 

 with the adhering papillae, springs away from the endo- 

 derm at this point, leaving a space into which the two 

 lateral mesodermic bands force their way. 



In this way a special anterior portion of the body-cavity, 

 praeoral and praeenteric, is produced, and is at first com- 

 pletely filled by a compact mass of rounded cells derived 

 from the mesodermic bands. 



The end of the body of the larva at which the adhering 

 papillae are placed of course corresponds to the tip of the 

 snout in Amphioxus. 



Just as Amphioxus burrows into the sand with its snout, 

 so the Ascidian larva fixes itself to the surface of a rock 

 or weed by its snout. The anterior or praeoral portion of 

 the body-cavity, of which we have just traced the origin, 

 is, and subsequently becomes in a still more pronounced 

 way, the cavity of the snout, or prceoral lobe. 



* The pericardium arises ventrally from the endodermic wall of the bran- 

 chial sac, and the heart is formed by an infolding of the dorsal wall of the 

 pericardium. 



