122 



THE HUMAN EMBRYO. 



Hypothetical Development of the Blastodermic Vesicle in Primates. 



As there exist no direct observations on the earliest stages of man, we can only 

 surmise what those stages may be. It is evident that there is a very precocious 

 development of the mesoderm, of the extra-embryonic ccelom, of the amnion, and 

 of the trophoderm, because these four features are found very marked in the ear- 

 liest known stages alike of man, apes, and monkeys. There are certain rodents 

 and insectivora in which these same peculiarities occur more or less emphasized 

 in the earliest stages of which we possess knowledge. If we utilize these data as 

 a basis, we can reconstruct the following hypothetical scheme of the earliest stages 

 in man. 



The accompanying diagrams (Figs. 68 and 69) represent three successive purely 

 hypothetical stages of the human ovum. They are all conceived to represent longi- 



Am.c. ' 



Ent. 



FIG. 68. Two DIAGRAMS TO ILLUSTRATE THE HYPOTHETICAL EARLY DEVELOPMENT OF PRIMATES. 

 Am.c, Amniotic cavity. Cae, Ccelom. EC, Ectoderm, in B, bearing the anlages of villi. Ent, Entoderm. Mes', 

 Somatic mesoderm. Mes.", Splanchnic mesoderm. Tro, Trophoderm. 



tudinal sections. In the first stage the ectoderm, EC, forms a moderate sized vesicle 

 and is already thickened. It should probably be conceived as consisting of an 

 inner distinctly cellular layer and an outer much thicker trophodermic layer which, 

 is thickest over what corresponds to the embryonic region. This special thickening 

 is marked Tro in diagram A. The entoderm, Ent, forms a small vesicle underlying 

 the thickened portion of the trophoderm. -The mesoderm, Mes, is well advanced in 

 its development and already contains the large extra-embryonic coelom, Coe, and is 

 therefore divided into one layer which surrounds the entoderm, and a second layer 

 which underlies the ectoderm. In other words, the splanchnopleure and somatopleure 

 are already differentiated. In the next stage (Fig. 68, B) there has been a growth, 

 the ovum has become larger, the trophoderm has increased in thickness, and in 

 the mass of thickened ectoderm overlying the yolk-sac there has appeared a cavity 

 the future amniotic cavity which is, of course, entirely surrounded by ectoderm. 

 The portion of the ectoderm on the under side of this cavity consists of a single 

 layer of cells which by assuming a cylindrical form constitutes the thickened area- 



