EARLY DEVELOPMENT OP JULUS TERRESTEIS. 239 



■| in. object-glass by Beck, and a No. 2 eye-piece by Zeiss. They form a com- 

 plete rather diagrammatic series up to the time of hatching. 



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



1 



1 5 



oil imm. Reichert. nucl. Nucleolus, nu. Nucleus, y. sp. Yolk-spherules. 



Fig. 3.6. — Section through dividing segmentation mass on second day- 

 (jig- Reichert's oil imm.) y. sp. Yolk-spherules, nu. Nucleus, rea.^ Nucleus 

 of second segmentation mass. 



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

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

 plasmic network. 



Fig. 17 h. — 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. {-^ Reichert's oil imm.) 



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

 mesodermal keel. ec. Ectoderm, en. Endoderm. nu. Nucleus. (Jg- 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. (Jg 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. (jL 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. seg.^ Furrow 

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

 (Zeiss, c.) 



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

 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. m. seg. Mesodermal segment. (Zeiss, d.) 



