EARLY DEVELOPMENT OF JULUS TERRESTRIS. 469 
3 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. 
Fie. 15.—Nucleus of ovarian ovum just before hatching. Drawn under 3}; 
oil imm. Reichert. xzcl. Nucleolus. nv. Nucleus. y. sp. Yolk-spherules. 
Fig. 16.—Section through dividing segmentation mass on second day- 
zs Reichert’s oil imm.) y. sp. Yolk-spherules. xz. Nucleus. xw.? Nucleus 
of second segmentation mass. 
Fic. 17 a.—Part of a section through first day ovum, showing protoplasmic 
network. (-'; Reichert’s oilimm.) y. sp. Yolk-spherules. p. netw. Proto- 
plasmic network. | 
Fic. 17 6.—Part of a section through second day ovum, showing network. 
p.netw. Protoplasmic network. 
Fic. 18.—Part of a transverse section through third day embryo, showing 
segmentation mass dividing to form blastoderm. 4/. Blastoderm cells. y. 4. 
Yolk hypoblast. 2a. Nncleus. (}; Reichert’s oil imm.) 
Fic. 19.—Part of transverse section on fourth day, to show formation of 
mesodermal keel. ec. Ectoderm. ez. Endoderm. zz, Nucleus. (,!; Rei- 
chert’s oil imm.) 
Fic. 20 a.—Part of transverse section through sixth day ovum, to show keel. 
ec. Kctoderm. m.%. Mesodermal keel. y. sp. Yolk-spherules. (Zeiss, D.) 
Fig. 20 46.—Isolated cells of the keel. (,'; 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. 6. Meso- 
dermal bands. (Zeiss, c.) 
Fic. 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. (5 Reichert’s oil imm.) 
Fic. 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. Stomodeum. — seg.' Furrow 
marking off the head segment. mm. seg. 1, 2, 3, &c. Mesodermal segments. 
(Zeiss, C.) 
Fie. 25.—Longitudinal section through cephalic section. s¢. Stomodzum. 
ec. Kctoderm. mes. Mesoderm. 1 seg. First segment. (Zeiss, D.) 
Fic. 26.—Endoderm cell from an embryo of same date when the mesenteron 
is being formed. (,}; Reichert’s oil imm.) 
Fic. 27.—Longitudinal section through the proctodzum in same embryo as 
Fig. 25. The mesenteron is just being formed. pr. Proctodeum. ez. Endo- 
derm. m. seg. Mesodermal segment. (Zeiss, D.) 
