THE EARLY DEVELOPMENT OF THE MAMMAL 399 



III. THE DEVELOPMENT OF THE EXTERNAL FORM 

 OF THE HUMAN EMBRYO 



The earliest phases in the development of the human organ- 

 ism, up to the formation of the primitive streak, are not known 

 at all, and it is not until the medullary plate stage is reached 

 that the structure of the human embryo is fully known. 

 Scarcely half a dozen embryos younger than this have been 

 described. It is clear, however, that the processes leading to 

 the formation of the embryonic layers, the amnionic cavity, 

 etc., are in general similar to those found among the rat, mouse 

 and bat. That is, the amnionic cavity is formed by delamina- 

 tion, between the continuous trophoblast and the embryonic 

 ectoderm. The endodermal vesicle is much smaller than the 

 ectodermal trophoblastic vesicle, leaving a wide space between 

 the two layers (Fig. 161). When the mesoderm forms it ex- 

 tends rapidly through the extra-embryonic region of the blasto- 

 dermic vesicle, one layer (somatic) applied to the inner surface 

 of the trophoblast, the other (splanchnic) applied to the outer 

 surface of the endodermal vesicle, so that the large cavity of the 

 blastodermic vesicle becomes, in effect, an extra-embryonic 

 ccelomic space. The layer of endoderm with the splanchnic 

 mesoderm is at first the yolk-sac, although of course entirely 

 yolkless, while the ectodermal trophoblast with the somatic 

 mesoderm forms the chorion. (Further reference to the chorion, 

 as well as to the amnion and the allantois, is deferred until a 

 later section.) 



The reader should note that students of mammalian develop- 

 ment use the term "ovum" to designate any early stage in 

 development. In this connection the "ovum" includes not 

 only the embryo proper, but also all of the associated struct- 

 ures of the blastodermic vesicle. 



As the embryo enlarges, its posterior end remains attached 

 to the inner surface of the trophoblastic wall by a mass of meso- 

 derm cells. This attachment is known as the body stalk or 

 belly stalk. Sections through an early embryo would give, 

 therefore, the appearance shown in Figs. 161, D; 179, A, B. 



