LIST OF ILLUSTRATIONS. 



PAGE 



1. Human Spermatozoa, {after Retains) 44 



2. Full-grown Human Ovum before Maturation, {after 11'. Nagel), 46 



3. Ovum of a Worm (Sagitta) with Two Pro-nuclei. Around Each Pro-nucleus is shown the Aster 



(after O. Hertwig), 5 2 



4. Ovum of a Rabbit, Seventeen Hours after Coitus, with the Pro-nuclei about to Conjugate {after Rein), 52 



5. Ovum of White Mouse. Beginning of the Conjugation of the Pro-nuclei {after Sobotta), S3 



6. Ovum of White Mouse. Conjugation of the Pro-nuclei, and Formation of the Segmentation Spindle 



{after Sobotta), • S3 



7. Ovum of White Mouse. First Segmentation Spindle with the Chromosomes of the Pro-nuclei still 



forming Two Distinct Groups {after Sobotta), 53 



8. Ovum of a Rabbit of Twenty-four Hours {after Caste), 55 



9. Ovum of a Snail (Limax campestris) during the First Cleavage. The Envelopes of the Ovum are 



not Drawn in {after E. L. Mark), 55 



10. Ovum of White Mouse. First Segmentation Spindle with Equatorial Plate of Chromosomes {after 



Sobotta), 5 6 



11. Ovum of White Mouse. First Segmentation Spindle {after Sobotta), 56 



12. Ova of White Mouse with Two Segmentation Spheres or Cells (after Sobotta), 57 



13. Ovum of a Bat (Vespertilio murina) with Four Segmentation Spheres (after van Beneden and Julin), 58 



14. Ovum of a Virginian Opossum, with Four Segments (after Emit Se/enka), 58 



15. Rabbit's Ovum of about Seventy Hours (after E. van Beneden), 59 



16. Young Blastodermic Vesicle of a Mole (after lt~. heafe), 59 



17. Sections through the Inner Mass of Blastodermic Vesicles of the Mole at Three Successive Stages 



(after IV. Heafe), r 6l 



18. Transverse Section through the Embryonic Shield of the Blastodermic Vesicle of a Dog of Eleven 



or Fifteen Days (Precise Age Unknown) (after Bonnet), 62 



19. Surface View of the Embryonic Shield of the Blastodermic Vesicle of a Dog of Thirteen to Fifteen 



Days (Precise Age Unknown) (after Bonnet), 63 



20. Central Portion of a Sheep's Blastodermic Vesicle of Twelve to Thirteen Days (after Bonnet), . . 64 



21. Blastodermic Vesicle of a Rabbit of Seven Days. Portion of the Mescderm of the Area opaca 



(after Kolliker), 64 



22. Germinal Area of a Guinea-pig at Thirteen Days and Twenty Hours, seen from the Under (Ento- 



dermal) Side, 66 



23. Transverse Section of a Mole Embryo (Heape's Stage H) (after IV. Heafe), - . 67 



24. Degenerating Tissue of the Notochord from the Central Portion of the Intervertebral Disc of a Cow's 



Embryo (after Leboucg), 69 



25. Surface View of the Embryonic Shield of a Dog Embryo, with Medullary Plate 69 



26. Cross-section of a Human Embryo of 1.54 mm. (after Count Sfee), 7° 



27. Section of a Young Cat Embryo and of the Uterine Wall to Which it is Attached. (Embryo No. 



398, Section 76) 7 2 



28. Transverse Section of a Rabbit Embryo of Eight Days and Two Hours, .... 73 



29. Three Diagrams of Embryonic Areas of Chicks to show the Growth of the Mesoderm (after Dnvai), 75 



30. Transverse Section of an Early Stage of an Axolotl (after Bellonci), 77 



31. Generalized Diagram of an Amniote Vertebrate Embryo, 7& 



32. Transverse Section from a Chick Embryo with about Eighteen Segments 80 



33. Section of a Very Young Cat Embryo. iTransverse Series 413, section 181), Si 



34. Diagrammatic Section of the Yolk of a Hen's Egg at an Early Stage to show the Relation of the 



Primitive Entodermal Cavity °5 



35. Wall of the Yolk-sac in the Region of the Area opaca of a Chick of the Second Day, 86 



