MORPHOLOGY OF BRAIN AND SENSE ORGANS OF LIMULUS. 93 



to median eye-uerve. r. m, e. t. Remnants to median eje-tube. r. ms. 

 Rostral mesoderm, s.l. Semicircular lobes := infundibulum ^ I'organ stratifie. 

 sf.ff. Stomodseal gfanglion. sL n. Stomodseal nerves, s. t. p. Strand con- 

 necting the nucleus to parietal eye-roots with the peduncle of optic ganglion. 

 t. e. c. Thin ectoderm between cerebral hemispheres, fh, r. Thickened ring 

 surrounding the cephalo-thorax. v. 5th ventricle, va. n. Vagus neuromeres 

 and nerves, v. s. Ventral surface. 



PLATE 3. 



Fig. 24. — Slightly diagrammatic view of the brain of an embryo in a stage 

 when the legs and from two to three pairs of gills are developed. Tlie draw- 

 ing is constructed from surface views and sections, and is intended to show the 

 relations of the various invaginations and the lines along which the imperfect 

 ectodermic folds (m. c.f., m.f. c. r.) advance to enclose the brain. 



Fig. 25. — Similar view of an older embryo, showing the greatly increased 

 cerebral hemispheres and the areas still uncovered by the advancing folds, 

 m. c.f. and m.f. or. 



Fig. 26. — Camera drawing of a cross-section through the brain in a stage 

 like that shown in Fig. 19. The section passes through the end of the line, 

 p. inf. 



Fig. 27. — Cross-section of a brain in a later stage than that shown in 

 Fig. 24, and in a plane that would pass through about the end of the line c. m. 

 of Fig. 24. 



Figs. 28 — 32.— Cross-section of the brain and nerve-cord in about the same 

 stage as that shown in Fig 24. 



In Fig, 28 the section plane passes through about the middle of the cere- 

 bral hemispheres (compare Fig. 24). 

 In Fig. 29, just back of the cerebral hemispheres. 

 In Fig. 30, just in front of the first pair of appendages. 

 In Fig. 31, through one of the lateral nerve-cords, just posterior to the 



second pair of appendages. 

 In Fig. 32, tlirough the ventral cords, a little back of the middle of the 

 third post-oral neuromcic. X 200. 

 Fig. 33. — Longitudinal section of a later stage than that of the preceding 

 series, passing through the base of the optic ganglion. It shows the ante- 

 rior wall of the invagination, and first differentiation of the primary olfactory 

 organ, x 200. 



Fig. 34.— Part of a longitudinal section in a little older stage than that of 

 the preceding series. The section shows the early stages of the lateral eye, 

 while it is in its primitive position close to the optic ganglion. 



