442 OUTLINES OF CHORDATE DEVELOPMENT 



relations in other forms, the student may be referred to Mar- 

 shall's " Physiology of Reproduction.") 



The very early stages in the history of the human ovum are 

 not yet known, but from comparisons with other similar forms, 

 and from the conditions of the youngest embryos known (see 

 Peters, Bryce-Teacher) the characters of the early human 

 blastodermic vesicle may be inferred with a high degree of 

 probability. It is entirely probable that the entire tropho- 

 blastic surface of the vesicle becomes trophodermic (Fig. 161), 

 and digests or erodes the uterine mucosa all around itself, when 

 it becomes attached to the uterine wall after entering the uterine 

 cavity. This attachment and implantation of the "ovum" 

 usually takes place on the anterior (upper) wall of the cavity, 

 between the openings of the oviducts, in what is known as the 

 fundus of the uterus. Attachments to other parts of the wall 

 are not infrequent, however, and do not affect the normality 

 of development. 



The "ovum" or vesicle apparently eats its way a short 

 distance into the mucosa, which closes behind it, and by de- 

 stroying the adjacent tissues becomes surrounded by a narrow 

 space filled with fluid extravasated blood and the products 

 of erosion (embryotrophe). This space is the rudiment of 

 what is later known as the intervillous cavity. The uterine 

 cells in this region then begin to proliferate rapidly, so that as 

 the blastodermic vesicle enlarges it remains covered with a 

 distinct layer of the uterine mucosa; this, as we shall see, is 

 the beginning of the decidua capsularis or reflexa. In some of 

 the Rodents the injury of the mucosa, when combined with 

 the presence of an internal secretion from the ovary (or corpus 

 luteum) serves as the stimulus to this proliferation (L. Loeb). 



The chorionic villi are formed very early and at first develop 

 all over the surface of the vesicle. Some of the villi simply 

 extend freely into the intervillous cavity, while others grow 

 across it and reach the undisturbed tissues of the uterine 

 mucosa, to which they become definitely attached as the 

 fixation villi. 



Before tracing further the history of the villi and the for- 



