308 ARTHUR WILLEY, 



this section is taken and the neuropore has in transverse 

 section an approximately round contour, and is quite simple. 

 Its lumen, which more anteriorly is reduced to a minimum, 

 gradually widens out until it becomes a transversely elongated 

 slit, as shown in the section figured. A large cell in the 

 dorsal wall of the neural tube in fig. 25 can be identified as 

 the otocyst, although at present it contains no pigment. 



The nerve-tube has therefore not yet commenced to swell 

 out in its anterior region into the remarkably voluminous 

 cerebral vesicle which appears later. At this stage it is chiefly 

 desired to call attention to the fact that at a region consider- 

 ably removed from its anterior extremity the neural tube, 

 though still simple, possesses a transversely elongated lumen. 



Stage II. — In embryos belonging to this stage the nerve- 

 tube still opens in front to the exterior by the neuropore. No 

 mouth is present, but the atrial involutions have put in their 

 appearance, in the form of the two well-marked longitudinal 

 grooves which I have previously described (No. I). No 

 stigmata have broken through. This stage also marks the 

 first appearance of pigment in the brain, while the otocyst has 

 attained its ventral position. In fig. 30 an intermediate stage 

 in the migration of the otocyst is shown, its position there 

 being lateral, on the right wall of the cerebral cavity. 



Figs. 26 — 29 represent transverse sections through the cere- 

 bral portion of the nervous system of an embryo belonging 

 to Stage II. Fig. 26 passes through the neuropore, and was 

 drawn with a higher power than the succeeding figures of 

 this series. In fig. 27 the section is taken a little behind the 

 neuropore, and the regularity of the circumference of the 

 neural tube is disturbed on the right side (left of the figure) 

 by a bluntly-pointed protuberance, which becomes still more 

 prominent as we pass backwards in the series. Meanwhile the 

 neural tube begins to show a tendency to divide itself trans- 

 versely into two portions; and, in fact, when we reach the 

 point from which the section shown in fig. 28 was taken, we 

 find that here the neural tube is double, and possesses in this 

 region two distinct lumina. 



