432 MARGARET ROBINSON. 



Fig. 38. — Transverse section through the anterior region of segment III 

 in Stage D. 



Fig. 39. — Transverse section tlirough the middle region of segment III in 

 Stage D. 

 Fig. 40. — Transverse section through segment IV in Stage D. 



Fig. 41.— Transverse section through the ventral part of segment V in 

 Stage E. 



Figs. 42, 43. — Transverse sections tlirough hindmost end of the embryo in 

 Stage D. 



Figs. 44 — 46. — Transverse sections through the optic segment in Stage E. 

 Fig. 44 is the most anterior. 



Fig. 47. — Transverse section through the anterior part of the deutero- 

 cerebrum in Stage E. The posterior ends of the optic lobes are cut through 

 in this section. 



PLATE 20. 



Fig. 48. — Longitudinal section througii the brain in Stage E. 



Fig. 49. — Transverse section through the posterior region of tlie deutero- 

 cerebrum in Stage E, showing the two ganglia of the antenuules with the 

 stomodseum between tiiem. 



Fig. 50. — Transverse section through the ventral part of the segment II 

 in Stage E, showing the antennulcs, the antenuulary ganglia (posterior part 

 of the deuterocerebrum) and the stomodaum. 



Fig. 51. — Transverse section through the ventral part of segment III in 

 Stage E, showing the antennary ganglion, the shell thickenings, labrnni, and 

 mouth. 



Fig. 52. — Transverse section through the ventral part of segment IV 

 (mandibular segment) in Stage E, showing mandibular ganglion with its 

 clcfi, labrum, etc. 



Figs. 53, 54. — Transverse sections through the antennary segment (behind 

 that shown in Fig. 52) in Stage E, showing the thickening of the dorsal 

 ectoderm and mesoderm cells (future cardiac cells) lying just below it. 

 X 238. 



Fig. 55. — Transverse section through the ventral part of the antennary 

 segment in Stage E, showing the first trace of the antennary gland. 



Fig. 55, a, b, c. — Three consecutive transverse sections through the 

 dorsal side of a stage a little older than Stage F, showing mesoderm cells 

 turning inwards to form the heart. 



