ANATOMY AND DEVELOPMENT. 



213 



The following table will help to make this matter 

 clearer. 



ORDER 



OF 

 OCCURRENCE. 



I. 



2. 



3- 



4- 



5- 



7- 

 8. 



ASCIDIAN. 



Gastrulation. 



Oval embryo with medullary 

 tube, neurenteric canal, 

 notochord, and mesoblast. 

 (Last two commencing.) 



Outgrowth of tail. 



Continued growth of tail. 



Formation of stomodosum and 

 atrial involutions. 



Escape from vitelline mem- 

 brane. 



Commencing perforation of 

 gill-clefts. 



Metamorphosis and commenc- 

 ing elongation of body 

 proper. 



AMPHIOXUS. 



Gastrulation. 



Oval embryo with medullary 

 tube, neurenteric canal, 

 notochord, and mesoblast. 

 (Last two commencing.) 



Commencing elongation of 

 body of embryo, and escape 

 from vitelline membrane. 



Continued elongation of em- 

 bryo. 



Formation of mouth, and com- 

 mencing perforation of gill- 

 clefts. 



Continued formation of gill- 

 clefts and outgrowth of tail 

 (i.e. post-anal section of 

 trunk). 



Formation of longitudinal atrial 

 involution. 



Metamorphosis. 



Of course the above table has no concern with the 

 actual time (hours and days) from the commencement 

 of the development at which such and such an event 

 occurs. The type of Ascidian referred to in the above 

 description is a simple Ascidian like Ciona or Phallusia. 



The above table also shows how the development of 

 the Ascidian and of Amphioxus moves along parallel 

 lines up to a certain point, and then at the time of the 

 outgrowth of the tail in the embryo of the former and the 

 hatching of the embryo of the latter, divergences set in. 



