EARLr DEVELOPMENT OF JULTJS TERRESTRIS. 239 



I in. object-glass by Beck, and a No. 2 eyepiece by Zeiss. Tliey form a com- 

 plete rather diagrammatic series up to the time of hatching. 



Fig. 15.— Nucleus of ovarian ovum just before hatching. Drawn under J- 

 oil imm. Reichert. raac/. Nucleolus. «». Nucleus, y. «?. Yolk-spherules.^ ° 



Fig. 16.— Section through dividing segmentation mass on second day- 

 (J5 Reichert "s oil imm.) y. gp. Yolk-spherules, m. Nucleus, nil? Nucleus 

 of second segmentation mass. 



Fig. 17 a. — Part of a section through first day ovum, showing protoplasmic 

 network. (jL Reichert's oil imm.) y. sp. Yolk-spherules, p. netw. Proto- 

 plasmic network. 



Fig. 17 b. — Part of a section through second day ovum, showing network. 

 p. netw. Protoplasmic network. 



Fig. 18. — Part of a transverse section through third day embryo, showing 

 segmentation mass dividing to form blastoderm, bl. Blastoderm cells, y. h. 

 Yolk hypoblast, nu. Nucleus. (Jj Reichert's oil imm.) 



Fig. 19. — Part of transverse section on fourth day, to show formation of 

 mesodermal keel. ec. Ectoderm, en. Endoderm. mt. Nucleus. (JL Rei- 

 chert's oil imm.) 



Fig. 20 a. — Part of transverse section through sixth day ovum, to show keel. 

 ec. Ectoderm, m. k. Mesodermal keel. y. sp. Yolk-spherules. (Zeiss, d.) 



Fig. 20 b. — Isolated cells of the keel. (J^ Reichert's oil imm.) 



Fig. 21. — Part of a transverse section through an ovum on the ninth day 

 early, to show thickened bands of mesoderm, ec. Ectoderm, mes. b. Meso- 

 dermal bands. (Zeiss, c.) 



Fig. 22. — Part of transverse section through ninth day embryo, to show 

 median portion between mesodermal bands, ec. Ectoderm, sp. m. Splanch- 

 nic mesoderm, som. m. Somatic mesoderm. (^-^ Reichert's oil imm.) 



Fig. 23. — Isolated cells from transverse section on ninth day, to show con- 

 nection between mesoderm and ectoderm. {-^ Reichert's oil imm.) 



Fig. 24. — Transverse section through part of an embryo on the ninth day 

 late, to show the mesodermal segments, stom. Stomodseum. sey.' Farrow 

 marking off the head segment. m. seg. 1, 2, 3, &c. Mesodermal segments. 

 (Zeiss, c.) 



Fig. 25. — Longitudinal section through cephalic section, st. Stomodseum. 

 ec. Ectoderm, mes. Mesoderm. 1 seg. First segment. (Zeiss, d.) 



Fig. 26. — Endoderm cell from an embryo of same date when the mesenteron 

 is being formed. (-jL Reichert's oil imm.) 



Fig. 27. — Longitudinal section through the proctodseum in same embryo as 

 Fig. 25. The mesenteron is just being formed, pr. Proctodseum. en. Endo- 

 derm. TO. seg. Mesodermal segment. (Zeiss, r.) 



