STOMODAEUA\ AND MEDULLARY TUBE 



Jast chapter we will revert to this question. In both Anne- 

 ;lids and Chordates there is formed, in connection with 

 this very narrow definitive blastopore, a tube of ecto- 

 dermal origin which at one end opens to the exterior, 

 at the other end leads into the archenteron through an 

 opening which is the former blastopore. In Annelids this 

 ectodermal tube is the stomodaeum, the outer opening 

 being the mouth, the inner the cardiac pore; in Chordates 

 it is the neural tube with the neuropore and the neuren- 

 teric canal. Cell-lineage investigations on Ascidians by 

 Van Beneden and Julin (1884), Castle (1896) and Conklin 

 (1905) have taught us that here the first rudiment of the 



■M 



i4^r^- 



Fig. 2. Gastrula-stage and formation of the medullary folds in 



Clavellina Rissoana. 



b. blastopore, m. medullary fold, n. neural plate. 



From KoRSCHELT and Heider (1893. p. 1274, 1275), 



after Van Beneden and Julin (1887). 



medullary plate originally surrounds the blastopore as a 

 ring, or, according to CASTLE, as a crescent, a conclusion 

 ■equally reached for Craniates by the brothers HertwiG 

 (1892) in their "Urmundtheorie"'. A comparison which 1 

 happened to make between the gastrula-stage of a Proto- 

 rstomian of which 1 was studying the development (1912) 

 and that of Ascidians, directed my attention to the fact 

 that a similar cell-ring as in the latter gives rise to the 

 neural tube, in Prostostomia represents the rudiment of the 

 •stomodaeum, which is equally an ectodermal tube leading 



