96 



COMPARATIVE PHYSIOLOGY. 



Holding the egg longitudinally, crack it across the center 

 transversely, gently and carefully pick away the shell and its 



e» 



Fig. 98.— Various stages in tlie development of the frog from tlie egg (after Howes). 

 1. The seginenting 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 oeneath. 5. The same, on appear- 

 ance 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 dividing more rapidly than lower. 

 12. Later phase of 11. 13. Longitudinal vertical section of IS. 14. Segmenting 

 ovum at blastopore stage. 15. Longitudinal vertical section of same. 13 and 15 

 X 10 (all others x 5). 16. Longitudinal vertical section of embryo at a stage later 

 than 14(1 x 10). tcc, nucleus; c.c, cleavage cavity; e^.epiblast; /./.yelk-bearing 

 lower-layer cells; W, blastopore; al, archenteron (mid-gut); A&, hypoblast; ms, 

 nndiSerentiated mesoblast; ch, notochord; n. a, neural ^erebro-spinal) axis. 



memhranes, when the blastoderm may be seen floating upward, 

 as it always does. It should be well examined in position, 



