378 OUTLINES OF CHORDATE DEVELOPMENT 



thin and easily ruptured; this is the cicatrix or stigma. It hag 

 been suggested that the rupture of the follicle may be due to 

 the continued accumulation of the liquor folliculi. However 

 this may be, when the follicle bursts the liquor flows out into 

 the periovarial cavity, carrying along the ovum, still surrounded 

 by the corona radiata. The fimbriaB of the oviduct are also 

 enclosed in this periovarial cavity, and through the ciliary 

 action of the epithelium covering these and lining the upper 

 part of the oviduct, and probably also through peristaltic con- 

 tractions of the oviducal walls, the ovum is carried through the 

 ostium and into the oviduct. During this passage the first 

 polar body frequently breaks through the chorion (zona pellu- 

 cida or radiata) and through the corona radiata as well, so that 

 it is entirely lost from the region of the ovum (mouse, Kirkham). 



Returning to the history of the follicle itself, we find it 

 undergoing very important changes, as the result of which it is 

 converted into the corpus luteum. The emptied follicle soon 

 becomes a nearly solid mass of cells, known as lutein cells, large 

 rounded cells containing quantities of pigmented granules or 

 lutein. The origin of these cells is somewhat uncertain; they 

 appear to be derived from the stratum granulosum cells of the 

 follicle, although they may come from the inner capsule of the 

 follicle (stroma cells). Their pigment is yellowish in man, 

 hence the name corpus luteum; in other Mammals it may be 

 pinkish (pig, rabbit), red (mouse), brown (sheep), etc. 



In cases of non-pregnancy following ovulation, the corpus 

 luteum is rapidly converted into fibrous connective tissue and is 

 absorbed, but when pregnancy follows, the corpus luteum 

 retrogresses very slowly and disappears only after parturition. 

 There is considerable evidence (Marshall, L. Loeb) that the 

 corpus luteum produces an internal secretion (hormone) of 

 great physiological importance in effecting the fixation of the 

 ovum to the walls of the uterus (implantation, see below). 



The general conditions determining the occurrence of ovula- 

 tion are unknown in most instances. In the lower Mammals 

 it is associated with a general physiological condition known as 

 cestrus or heat, which may possibly itself be determined by 



