MORPHOLOGY OF BRAIN AND SENSE ORGANS OF LIMULUS. 63 



from a little earlier stage than that in fig. 24, and illustrate 

 the continuity of the invagination cavities. There were four- 

 teen sections in the series, of which the fourteenth is shown 

 in fig. 35 ; the eleventh, ninth, fifth, and second, in the four 

 succeeding figures. The other sections of the series showed 

 nothing of interest, and so were not represented. The 

 position of the sections is shown by the dotted lines in fig. 24. 

 The invaginations do not persist very long, and the method of 

 disappearing varies somewhat in the difi'erent parts. In fig. 39 

 the ectoderm has already broken away from the anterior wall 

 of the invagination, and is now pushing its way backward 

 over the oi)tic ganglion, in order to unite with the thin ecto- 

 derm behind it. The anterior wall (a. w.), which assumes a little 

 darker colour, finally fuses with and forms a part of the 

 definitive optic ganglion. A section through this region in a 

 little later stage (fig. 33) still shows traces of the anterior wall 

 of the fold (a. w.). The figure also shows the characteristic way 

 in which the thick layer of ectoderm, constituting the ganglion, 

 becomes tilted over, so that its inner surface, where the 

 medullary substance is just appearing, is turned outward. 

 Another section of the same stage, across the distal end of the 

 optic ganglion, and showing the first traces of the convex eye, 

 is shown in fig. 44. 



On the median side of the optic ganglion, to go back to the 

 stage we started with, the invagination is already quite faint 

 (figs. 37 and 38, i. v.), and in the next stage disappears, both walls 

 forming brain tissue. The same is true of the semicircular 

 lobes (fig. 36, i. v. sL), although the invagination cavity and its 

 walls can be distinguished as such for a considerably longer 

 period than in the optic ganglion. In the median section (fig. 

 35) one sees the median eye-tube cut lengthwise; the relatively 

 small commissure uniting the halves of the brain {m. c), 

 and, at the tip of the rostrum, a great mass of transitory 

 tissue {r.m. s.), the nature of which could not be determined. 

 In fig. 26 a cross-section of the anterior margin of the cephalic 

 lobes of the same stage shows the extent of the two invagina- 

 tions for the semicircular lobes. 



