430 MARGARET ROBINSON. 



PLATE 16. 



Fig. a. — Stage A, Young embryo showing the blastoderm, which does 

 not as yet completely surround the yolk. 



Tig. B'. — Stage B'. Late gastrula stage iu which the blastopore is closed, 

 and the optic and antennal thickenings are definitely apparent. 



Fig. C. — Stage C. Embryo with nauplius appendages. 



EiG. D. — Stage D. Embryo with zosea appendages. 



EiG. E. — Stage E. Embryo in which the appendages more or less follow 

 the outline of the body. 



EiG. F. Stage E. Earliest stage in which the embryo is free from the 

 shell. On account of its bifid appendages this might be called the Mysis 

 stage. 



EiG. 1. — Longitudinal section through the ovum, younger than Stage A, 

 showing cells within the yolk as well as on the ventral surface. X 238. 



EiG. 2. — Longitudinal section through Stage A, showing the yolk as yet 

 incompletely surrounded by the blastoderm, x 238. 



EiG. 3. — Transverse section through the lateral thickenings in Stage B. 

 div. Dividing cells. 



EiG. 4. — Transverse section through the lateral thickenings (behind Eig. 3) 

 in Stage B. 



Fig. 5. — Transverse section through the anterior region of the caudal 

 thickening (front end of the groove) in Stage B. x 238. 



Fig. 6. — Transverse section through the posterior thickening at the middle 

 of tlie groove in Stage B. X 238. 



Fig. 7. — Transverse section through hind end of the caudal thickening in 

 Stage B (behind the groove), x 238. 



PLATE 17. 



Fig. 8. — Transverse section through the optic thickenings in Stage B'. 



Fig. 9. — Transverse section through Stage B', just behind the optic 

 thickenings. 



Fig. 10. — Transverse section through Stage B', taken midway between tiie 

 optic thickenings and the caudal thickening. 



Figs. 11 — 15. — Transverse sections through the caudal thickening in 

 Stage B'. 



Fig. 10. — Transverse section through the anterior part of the optic seg- 

 ment in Stage C, ot. Optic thickening. 



