STRUG rUKE AND DEVELOPMENT OF REISSNER’s PIBUE. 75 
The terminal ping still retains a postei’o-dorsal insertion. At 
this stage the canalis centralis is seen to have increased 
considei’ably in size, bnt iinmediatelv anterior to the extremity 
of the filnm terminale it appears to become more shallow, 
then deepens and widens into the sinus terminalis. 
Thus, in this particular specimen, the terminal sinus appears 
conical rather than ovoid in shape. It is probable, however, 
that the posterior portion of its wall, constituted by the 
meningeal sheath, has collapsed somewhat, for in a slightly 
larger specimen (52 mrn.) the arrangement of the various 
structures is precisely similar, excepting that the meningeal 
portion bulges upwards and outwards, dome-like. It will be 
noticed that this meniugeal portion of the wall of the sinus 
now lies posteriorly instead of postero-dorsally. 
In older larva) the neural tube has apparently outgrown the 
supporting notochord and has become turned down behind it. 
In fig. 52 the posterior end of the spinal cord of a 75 mm. 
ammocoete is shown partly bent, the bend occurring at the 
anterior end of the sinus terminalis, while iu the oldest 
larva that I have examined (105 mm.), the terminal neural 
pore has become ventrally directed, and lies below the level 
of the notochord (figs. I f, 54). 
Unfortunately, in none of these older am mocoetes examined 
(all of which were preserved entire) has the fibre remained in 
its normal position, but it is broken und ends in a tangle in the 
sinus terminalis. 
Such tangles have been found in the terminal sinus of 
specimens of 05 mtn. (fig. 55), 75 mm. (fig. 52), 90 mm. (fig. 
15), 95 mm. and 105 mm. (figs. 14, 54). The amount of 
retracted fibi'e varies considerably in the different specimens, 
forming in the cases of the 90 mm. and 95 mm. ammocoetes 
conspicuous mas.ses of spirally twisted fibre that almost fill 
the terminal sinus. 'I'he condition of the fibre iu the 05 mm. 
specimen is noteworthy, for in this case the recoil seems to 
h.ave pulled into the terminal sinus some of the fibrous 
meningeal tissue. The terminal plug cannot be distinguished, 
being overlain, presumably, by the tangle of Reissner’s fibre. 
