THE MATURATION OF THE OVUM. 47 



Graafian follicle fills up the space of the same, constituting a mass which is 

 known as the corpus luteum on account of its yellow color. The most character- 

 istic elements of this structure are the large cells which contain the pigment. 

 Each cell has a rounded nucleus and a large protoplasmic body, which is also 

 more or less rounded in form. The lutein granules are in these cells. The 

 function of the corpus luteum was long entirely unknown. Recently the theory 

 has been suggested by Born that these cells exert an influence upon the uterus 

 by which it is prepared to receive the ovum. This influence may be suggested to 

 act by means of a chemical substance produced by the lutein cells and added to 

 the blood, which then affects the uterus. There are some experimental observa- 

 tions tending to prove the correctness of this theory. 



The brilliant color of the corpus luteum is especially characteristic of man, 

 and has determined the name of the structure. In sheep the pigment is pale 

 brown, in the cow dark orange, in the mouse brick red, in the rabbit and pig 

 flesh-color. 



The Maturation of the Ovum. 



Maturation is the term applied to the series of changes by which the fully 

 grown egg-cell is transformed into a true female sexual element. Viewed exter- 

 nally in the living ovum, the process manifests itself chiefly by the separating off 

 of one, or usually two, small bodies of protoplasm, each of which contains some 

 nuclear material. These small bodies are generally known by the name of polar 

 globules. They take no further part in the development, ultimately disintegrate, 

 and are lost. The remaining ovum is capable of impregnation. It is now 

 known that the production of the polar globules is the result of a special form of 

 cell division, which we term the "reduction division." When the first polar 

 globule is formed, the egg-cell divides into one very large cell and a second very 

 small one. When the second polar globule is formed, the larger of the cells again 

 divides, producing a second small cell and a new large one. This large one is the 

 true female element. When an ovum is about to mature, its nucleus moves 

 nearer that point of the surface which may be regarded as the center of the so- 

 called animal pole, or region of the ovum, which contains most of the protoplasm 

 and less of the yolk material. During the migration of the nucleus, the cell as a 

 whole usually contracts so that a space appears between it and the zona radiata. 

 Concerning the force that moves the nucleus we have no definite knowledge. 

 When near the surface, the nucleus as such disappears. Older writers supposed 

 that it was lost altogether, but we now know that the disappearance of the 

 nucleus is only apparent, not actual, being in reality a metamorphosis. It is 

 probable that the first step is the discharge of the nuclear fluid into the surround- 



