io^ 



THE ANCESTRV 6f VERTEBRATES 



less developed and leave an opening, the anus. Comparing 

 my description with that of former investigators it will be 

 noted that, keeping strictly to the facts, I yet present them 

 in a somewhat different way: 1 do not let the medullary 

 folds finish halfway along the length of the blastopore slit 

 but only in closing leave an opening over the rear end of 

 the blastopore, the anus. Accordingly one can, retracing 

 the medullary canal, not only pass through the neurenteric 

 canal into the archentcron . but also through the anus to 

 the outside, this being nowhere prevented by a coalescence 



Fig. 42. Transverse sections through the blastopore of an egg 

 of Amblystoma punctatum, where the medullary folds 

 just close over it, after Morgan, 1890. 

 a in front of the blastopore, b and c through anterior 

 half, d and e through rear end (anus). 



of the two medullary folds across the middle of the blasto- 

 pore, as several investigators are inclined to assume. 



Now in a longitudinal section (fig. 8, plate 111) the 

 blastopore (bl.=^p. near.) and the anus \a) are easily 

 distinghuishable from one another. The blastopore becomes 

 the neurenteric canal or, perhaps better, the neurenteric 

 pore (poms neurentericus), as I prefer to call it henceforth. 

 Entering the anus, one can pass through the neurenteric 

 pore into the archenteron. The anterior part of the neuren- 

 teric pore, however, becomes — and is already in fig. 8 — 

 virtual, the medullary folds applying themselves behind so 

 closely to one another that the lumen of the medullary 



