CHAPTER VI 



THE EARLY DEVELOPMENT OF THE MAMMAL. THE 

 MAMMALIAN EMBRYONIC MEMBRANES AND 



APPENDAGES 



PAGE 

 INTRODUCTION 368 



I. THE EGG AND ITS FORMATION 371 



1. The Reproductive Organs of the Female 371 



2. The Ovum and its Ovarian History 372 



3. Maturation 375 



4. Ovulation 377 



II. FERTILIZATION AND THE EARLY PHASES OF DEVEL- 



OPMENT 379 



1. Fertilization 379 



2. Cleavage 380 



3. The Blastodermic Vesicle 353 



A. The Growth of the Blastodermic Vesicle 384 



B. The Formation of the Embryo and the Embryonic 

 Layers 384 



III. THE DEVELOPMENT OF THE EXTERNAL FORM OF 



THE HUMAN EMBRYO 399 



IV. THE EMBRYONIC MEMBRANES AND APPENDAGES OF 



THE EUTHERIAN MAMMALS 417 



1. Implantation 421 



2. The Amnion and Chorion 424 



3. The Yolk-sac 431 



4. The Allantois 434 



5. The Placenta 437 



WE shall not undertake, in the present chapter, to give an 

 account, however brief, covering the whole embryonic history 

 of a Mammal. We shall rather attempt to describe certain 

 phases or aspects of mammalian development, selected on 

 account of their interest or importance for the general student. 

 A\ e shall give first a description of the mammalian ovum, its 

 formation, the early processes of cleavage and the formation 



368 



