436 
MESSRS. F. M. BALFOUR AND W. N. PARKER ON THE 
v.sh. 
Vascular sheath between the 
z.r. 
Outer striated portion of invest¬ 
hyaloid membrane and the 
ing membrane (zona radiata) 
vitreous humour. 
of ovum. 
v.th. 
Vesicle of the thalamencephalon. 
z.r\ 
Inner noil-striated portion of in¬ 
X. 
Groove in epiblast, probably 
vesting membrane of ovum. 
formed in process of harden¬ 
I. 
Olfactory nerve. 
ing. 
II. 
Optic nerve. 
y- 
Yolk. 
III. 
Oculomotor nerve. 
z. 
Commissure in front of pineal 
V. 
Trigeminal nerve. 
gland. 
VIII. 
Facial and auditory nerves. 
Explanation of Figures. 
PLATE 21. 
Figs. 1—4. Different stages in the segmentation of the ovum. 
Fig. 1. Ovum with a single vertical furrow, from above. 
Fig. 2. Ovum with two vertical furrows, from above. 
Fig. 3. Side view of an ovum with ti completely formed blastodermic 
disc. 
Fig. 4. The same ovum as fig. 3, from below, showing four vertica 
furrows nearly meeting at the vegetative pole. 
Figs. 5-10. External views of embr} T os up to time of hatching. 
Fig. 5. Embryo, 3‘5 millims. long, third day after impregnation. 
Fig. 6. Embryo on the fifth day after impregnation. 
Fig. 7. Posterior part of same embryo as fig. 6, showing tail swelling. 
Fig. 8. Embryo on the sixth day after impregnation. 
Fig. 9. Embryo on the seventh day after impregnation. 
Fig. 10. Embryo on the eleventh day after impregnation (shortly before 
hatching). 
Fig. 11. Head of embryo about the same age as fig. 10, ventral aspect. 
Fig. 12. Side view of a larva about 11 millims. in length, shortly after hatching, 
Fig. 13. Head of a larva about the same age as fig. 12, ventral aspect. 
Fig. 14. Side view of a larva about 15 millims. long, five days after hatching. 
Fig. 15. Head of a larva 23 millims. in length. 
Fig. 16. Tail of a larva 11 centims. in length. 
Fig. 17. Transverse section through the egg-membranes of a just-laid ovum. 
We are indebted to Professor W. K. Parker for figs. 12, 14, and 15. 
