294 CHORD AT A. 



presence of the atrium, Ascidia can be directly compared 

 with the other class of the Atriozoa. 



The conclusions drawn from these characters have, how- 

 ever, an ample corroboration in the development. 



The eggs are laid into the atrium, in which they are 

 fertilised and pass their early stages. Later, the larva is 

 free-swimming and pelagic. 



The segmentation is total and nearly equal, producing a 

 hlastula which is invaginated to form a gastrula. The 

 gastrula elongates, and the blastopore comes to lie in a 

 postero-dorsal position in relation to the adult axes. From 



Fig. 206. — Transverse Section through Embryo of an 



ASCIDIAN. 

 (After Delage.) 



Neural Groove. 



Notochord. 



Epiblast. 



/ 



Archenteron. 



the blastopore forwards to the anterior end of the gastrula 

 the median dorsal line of cells becomes the dorsal nervous 

 systein, which is at first dermic, but it is transformed into a 

 long dorsal nerve-tube by invagination proceeding from 

 behind forwards. The front end of the tube, called the 

 neuropore, is open, and the posterior end, leading through the 

 blastopore into the archenteron^ is known as the neiirenteric 

 canal. Meanwhile the hypoblast has been developing. 

 The hypoblastic cells lying in the mid-dorsal line immedi- 

 ately below the neural tube become pinched off from the 

 rest to form a long rod-like body, the notochord. Laterally 

 to this organ are paired pouchings of the hypoblast which 

 give rise to the mesoblast or third embryonic layer. Their 

 lumen is soon lost, and the mesoblast comes to lie as a pair 

 of lateral masses of cells between epiblast and hypoblast. 



We now have the typical chordate larva or Chordula, con- 

 sisting of an elongated body, with a long dorsal nerve-tube, 

 opening anteriorly to the exterior, posteriorly into the arch- 

 enteron, a median dorsal notochord separated from the 



