116 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 preco- 

 cious development of the mesoderm, of the extra-embryonic coelom, of the 

 amnion, and of the trophoblast, because these four features are found very 

 marked in the earliest 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. 53 and 54) represent three successive 



Fig. 53. — Two Diagrams to Illustrate the Hypothetical Early Development of Primates 



Am.c, Amniotic cavity. Coe, Coelom. Ec, Ectoderm, in B, bearing the anlages of villi. Ent, Entoderm. 



Mes'y Somatic mesoderm. Mes'^ , Splanchnic mesoderm. Tro, Trophoblast. 



purely hypothetical stages of the human ovum. They are all conceived to 

 represent longitudinal sections. In the first stage the ectoderm, Ec, forms a 

 moderate sized vesicle and is already thickened. It should probably be con- 

 ceived as consisting of an inner distinctly cellular layer and an outer much 

 thicker trophoblastic layer which is thickest over what corresponds to the em- 

 bryonic region. This special thickening is marked Tro in diagram A. The 

 entoderm, Ent, forms a small vesicle underlying the thickened portion of the 

 trophoblast. The mesoderm, Mes, is well advanced in its development and 

 already contains the large extra-embryonic ccelom, Cce, 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 dififerentiated. In the next stage (Fig. 53, B) there has been a growth, 

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



