but a little to the left of this, Pig. 23. 3y the time the 

 shifting of the vesicle has proceeded soraev/hat further than 

 is shown in Fig. 11, the string of cells, which is now solid, 

 has been carried up to the median plane. 



Very shortly after it has reached its definitive 

 position, a lumen appears in the centre of the rudiment 

 throughout its entire length, and around this the cells be- 

 come arranged into a one-layered epitheliiim. Pig. 27, d_ and 



By following the course of development up to this 

 point, it is seen hov/ an epithelial tube does actually arise 

 from free mesodermal cells, a thing which Hjort has charac- 

 terized as most improbable. In criticising Seeliger's view 

 of the origin of the neuro-hypophyseal system in Clave lina, 

 this author says, (8: page 602) "die Wahrscheinlichheit da- 

 fur, dass ein Ganglion und ein epitheliales Rohr sich aus 

 zusammengehauf ten Mesodermzellen bilden sollte, scheint mir 

 so gering zu sein, wie fur die Auffassung Herdman's dass die 

 Innere Blase der Knospenanlage einen solchen Ursprung habe . " 



About the time that the peribranchial cavity is 

 completely cut off from the inner vesicle, the anterior ex- 

 tremity of the dorsal tube fuses with the dorsal wall of the 

 branchial sac, an opening breaks through, and the lumen of 

 the tube is put into communication with the branchial cavity. 

 The posterior end of the tube abuts against the anterior 

 wall of the cloaca, but neve-.r opens into the latter, in con- 

 trast with the condition found in Botryllus and many other 



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