MORPHOLOGY OF BRAIN AND SENSE ORGANS OF LIMULUS. 65 



delamination a thin layer of superficial ectoderm^ which pro- 

 bably unites with the medullary folds to form the continuous 

 layer that finally covers the ventral cords. 



The margin of the ectodermic fold advances forward and 

 medianly over the cerebral hemispheres^ as shown by the line 

 m. c. f.f fig. 24. At the same time the narrow bridge of ecto- 

 derm leading to the pineal eye- tube is drawn backward, dimin- 

 ishing the clear triangular area behind it. Its anterior lip 

 then unites with the advancing fold on the cerebral hemi- 

 spheres, so that the surface of the hemisphere, not yet covered 

 by the ectoderm, has a contour something like that shown in fig. 

 25, c. h. The cross-bar of the ectoderm is finally converted, by 

 the union of its anterior and posterior lips, into a tube opening 

 by a small round pore immediately over the proximal end of 

 the median eye-tube. This pore, which I shall call the anterior 

 neuropore [n.p.), leads directly into the median eye-tube and 

 through the cross-tube into all the cavities of the fore-brain. 

 The lips of this pore also represent the last point of 

 attachment of the fore-brain with the surface ecto- 

 derm. 



Meantime the cerebral hemispheres have increased 

 rapidly in size. From the very start each hemisphere shows a 

 distinct separation into two lobes — a posterior one, drawn 

 out into long sharp points, and an anterior lateral lobe. 

 Both lobes are separated from each other by a deep fissure, 

 in the angle of which lies the slender fibrous peduncle on 

 which the hemisphere is supported (fig. 25, p. ch.). The 

 shape of the hemispheres in the second larval stage is accu- 

 rately shown in fig. 47. The fore-brain region is drawn from 

 a wax-plate model, the rest from dissected specimens. The 

 posterior lobes are not so slender and pointed as at an earlier 

 stage, and both lobes show the first traces of the convolutions 

 so characteristic of the brain at a later period. The hemi- 

 spheres have united with each other along the median line, 

 and the anterior lateral lobe is beginning to grow around on 

 to the under side of the brain. Two cross-sections of the brain in 

 this stage are shown, one through about the middle of the hemi- 



VOL. 35, PART 1. NEW SEK. E 



