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dif closed by the medullary folds, both took their way through 
e 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, 
Amphibien, Störe, Plagiostomen), bei denen Darm- und 
Nervenrohr ein zusammenhängendes Rohr darstellen, lässt 
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 erfüllt, 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 into 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 the stomodaeum of Protostomia. In the 
development of both groups the border of the blastopore, 
originally wide and large, contracts to a very narrow op@- 
ning which 1 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 
