195 
last 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 se 
at the other end leads into the archenteron through a 
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 
Fig, 2. Gastrula-stage pend Naden pl an medullary folds in 
a Rissoa 
b. blastopore, m. gen ba n. neural plate. 
From KORSCHELT and HEIDER (1893, % 1274, 1275), 
after VAN BENEDEN and JULIN (1887). 
pend plate her geler 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 l 
happened to make between the gastrula-stage of a Proto- 
stomian 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 
