CONTENTS 



Introduction 11 



I. Preliminary data. The ovarian egg. The testicular aperm. Mating. 14 



II. The first day. Stage 1. Fertilization. The tubal ovum 24 



III. The second day. Stage 2. The first cleavage. The extrusion of yolk. 28 



IV. The third day. Stage 3 to 8 31 



Stages 3 and 4. The second cleavage. 



Stages 5 and 6. The third cleavage. 



Stages 7 and 8. The fourth cleavage. 

 V. The fourth day. Stages 9 to 12 38 



Stage 9. The unilaminar blastocyst. 



Stage 10. The endoderm mother-cells. The new polarity 



Stages 11 and 12. The primitive endoderm. The medullary 

 plate. 



Stage 12. The beginning of expansion of the vesicle. 

 VI. The fifth and sixth days. Stages 13 and 14 4? 



Stage 13. The definitive endoderm. 



Stage 14. The bilaminar blastocyst. The uterine glands 

 VII. The seventh day. Stages 14 to 17 52 



Stages 14 and 15. The late bilaminar blastocyst. 



Stage 16. The primitive streak. 



Stage 17. The mesodermal crescent. 

 VIII. The eighth day. Stages 18 to 23 54 



Stage 18. Hensen's node and the primitive groove. 



Stage 19. Elongation of the medullary plate. 



Stage 20. The mesodermal rim. 



Stage 21. The notochord and the medullary groove. 



Stage 22. The first somites and the parietal mesoderm. 



Stage 23. The coelomic cavities and the subcephalic fold. 



IX. The ninth day. Stages 24 to 26 60 



The first third of the ninth day. 



Stage 24. The heart tubes and the first blood vessels. The 

 first sensory anlagen. The nephrogenic ridge. The first 

 branchial pouches. The hypophyseal plate and the pharyngeal 

 membrane. Proamnion. Differentiation in the medullary 

 plate and the neural crest. Miscellaneous details. Compara- 

 tive notes. 

 The second third of the ninth day. 



Stage 25. First contact of the neural folds. Fusion of the 

 hearts in the bulbar region. Origin of the sinus venosus. 

 Origin of the pharyngeal floor. Lung anlngen. Miscellaneous 

 details. Comparative notes. 



