STUDIES ON THE PROTOCHORDATA. 303 



What, then, is this pore leading from the stomodseura into 

 the neural tube? My answer is that it is the neuroporus. 

 We have already mentioned the fact, which was originally 

 determined by Kowalevsky, that the anterior opening to the 

 exterior of the medullary canal closed up before the formation 

 of the mouth. We now see that some time after the forma- 

 tion of the mouth — in fact, as soon as it begins to be func- 

 tionally active and to take in water, which does not occur 

 until after the fixation of the larva the neuroporus opens out 

 again, but this time into the stomodseum. 



The primary closure of the neuroporus was therefore only 

 temporary, and comparable to what occurs in the case of many 

 blastopores and other organs which become temporarily solid — 

 such, for instance, as the oesophagus of the Selachians. What- 

 ever may be the actual cause of the temporary closure of the 

 neuroporus in Ascidians, it is perfectly plain that its per- 

 sistence through the period during which it is closed would be 

 of no service to the embryo or larva, because the development 

 up to the time of the formation of the mouth takes place in- 

 side the egg-follicle, and when the mouth does appear it does 

 not at first open directly to the exterior, but is covered over 

 by the so-called testa. 



This is a quite different state of things from what we find in 

 Amphioxus, where the neuroporus does not undergo this 

 temporary primary closure; but then the embryo of Am- 

 phioxus leaves the follicle precisely at the stage in which, with 

 the above-named simple Ascidians, the neuroporus closes. 

 In Clavelina the neuropore remains open somewhat longer 

 than in the simple Ascidians mentioned above. 



Fig. 13 represents a sagittal section through a young fixed 

 Ciona of this stage, drawn with a low power to explain the 

 topographical relations of the various parts. The neural tube, 

 or neuro-hypophysial tube, as it may now be indifi^erently 

 called, is seen to open into the buccal cavity in front. Fig. 12 

 shows a portion of the neuro-hypophysial tube from the same 

 section, but drawn under a much higher power (Zeiss J, 

 water immersion). It commences anteriorly as a well-marked 



VOL. 35, PART 2. NEW SER. Y 



