3o6 



CHORD AT A. 



Development. — The eggs are shed through the atriopore to the 

 exterior, where they are fertiUsed. Segmentation is total and equal and 

 results in a blastula which in its turn is converted into a gastrula by 

 archiblastic invagination. The gastrula then elongates, the blastopore 

 taking up a postero-dorsal position. 



The epiblast then invaginates along the mid-dorsal Une to form a 

 nerve- tube and the hypoblast gives rise to a median dorsal notochord 

 and paired lateral mesoblastic sacs. In this manner is produced a 

 chordula larva practically similar to that of Ascidia. The main dis- 

 tinction lies in the origin of the mesoblast. Instead of a single pair of 

 somites which rapidly become a pair of solid mesoblastic masses, event- 

 ually breaking up into scattered cells, there are in Amphioxiis a great 

 number of somites, each of which has a definite coelomic cavity. It is 



P'ig. 220. — Transverse Sections through Young Amphioxus, 

 SHOWING Developing Atrium. 



(After Lankester and Willey, Boveri, and others.) 



Sclerotome. 

 Myotome 



Sclerotome. 



Gonotome. 



Metapleural 

 Cavity. 



Atrium. 

 Ventral Muscle*«*- 

 said that one pair of pre-oral somites arise from the anterior end of the 

 .archenteron, a second pair behind these, called the collar-sacs^ and a 

 third pair at the posterior end laterally to the blastopore. The pre- 

 oral pair form the head-cavity (right) of the larva and the pre-oral pit 

 (left). Each of the collar-somites divides into a dorsal portion, which 

 forms the first myomere muscle, and a ventral part forming the meta- 

 pleural cavity. Lastly, the posterior somites divide up to form a great 

 number of mesoblastic somites: so far as is known, they alone are 

 found in Ascidia. 



The three pairs evidently correspond to the three archimeric seg- 

 ments oi Balanoglossus d.xi^ the other Archico^loniata^ and the metameric 

 segmentation of the Chordata is clearly produced by secondary segmen- 

 tation of the posterior segment or opisthomere. 



