8 THE ANCESTRY OF VERTEBRATES 



dif closed by the medullary folds, both took their way through 

 the anus. Similar are the views of MORGAN (1890). 



Another suggestion had been made cursorily by KOWA- 

 LEWSKY (1877, p. 201), who together with GOETTE (1869, 

 p. 115) has discovered the canalis neurentericus, that 

 curious connection between the medullary tube and the 

 archenteron. He wrote : "Die sonderbare Bildung des Nerven- 

 systems bei den Embryonen vieler Wirbelthiere {Amphioxus, 

 Amphibian, Store, Plagiostomen), bei denen Darm- und 

 Nervenrohr ein zusammenhangendes Rohr darstellen, lasst 

 uns vermuthen, dass vielleicht solche Thierformen existirten 

 Oder auch existiren, welche ein dem Nervenrohr der Wir- 

 belthiere homologes Rohr besitzen, obgleich dasselbe eine 

 andere Function erfullt, dass es z. B ein Theil des Darm- 

 canals sei." This idea, however, was not worked out further 

 by KOWALEWSKY. He only mentions in this connection the 

 U-shaped alimentary tract of Bryozoa; afterwards he has 

 made no further reference to the subject. 



The gist of my theory is the supposition that indeed the 

 neural tube, as KOWALEWSKY once suggested, has been a 

 part of the alimentary tract, that it corresponds to the ecto- 

 dermal part of the latter in Invertebrates and that it is the 

 homologue of nothing else than the stomodaeum of those 

 animals which Hatschek took together as the Zygoneura 

 and which Grobben named the Protostomia. In this group 

 the blastopore of the gastrula becomes the definitive in- 

 gestion-opening, not the mouth, since round the blastopore 

 the ectodermal stomodaeum invaginates and only the exterior 

 opening of the latter is the mouth, while the blastopore 

 is found in the inner opening, leading in^o the entodermal 

 stomach., which opening 1 (1917, p. 1267) have proposed 

 to call the cardiac-pore. 



Medullary tube and stomodaeum. — There are indeed many 

 points of agreement between the medullary tube of chor- 

 date embryos and ihe stomodaeum of Protostomia. In the 

 development of both groups the border of the blastopore, 

 originally wide and large, contracts to a very narrow ope- 

 ning which I should like to call the definitive blastopore 

 and not, as some do, the rest of the blastopore, since in 

 my opinion the blastopore is the mouth of the gastrula 

 and only the latter stage can be called the gastrula, irre- 

 spective of whether the contraction of the blastopore pro- 

 ceeds in an eccentric or in a concentric manner. In the 



