THE DEVELOPMENT OF THE EMBRYO ITSELF. 91 
ments or pictorial representations, would be the examination of 
a setting of eggs day by day during their development under a 
: eee 
Fia. 102.—Various stages in the development of the waa from the egg (after Howes). 1. The 
segmenting ovum, showing first cleavage furrow. 2. Section of the above at right angles 
to the furrow. 3. Same, on appearance of second furrow, viewed slightly from above. 
4. The latter seen from beneath. 5. The same, on appearance of first horizontal furrow. 
6. The same, seen from above. 7. Longitudinal section of 6. 8 and 9. Two phases in 
segmentation, on appearance of fourth and fifth furrows. 10. Longitudinal vertical section 
at a slightly later stage than the above. 11. Later stage. Upper pigmented pole dividin; 
more rapidly than lower. 12. Later phase of 11. _ 13. Longitudinal vertical section of 12, 
14. Se; Poets ovum at blastopore stage. 15. Longitudinal vertical section of same, 
18 and 15 x 10 (all others x 5). 16. Longitudinal vertical section of embryo at a stage 
later than 14 (1 x 10). mc, nucleus; c.c, cleavage cavity ; ep, epiblast ; 1. 1, yelk-bearing 
lower-layer cells ; bl, blastopore ; al, archenteron (mid-gut) ; hb, hypoblast ; ms, undiffer- 
entiated mesoblast ; ch, notochord ; 7. a, neural (cerebro-spinal) axis. 
hen. This is a very simple matter, and, while the making and 
mounting of sections from hardened specimens is valuable, it 
may require more time than the student can spare; but it is 
neither so valuable nor so easily accomplished as what we have 
indicated ; for, while the lack of sections made by the student 
