36 THE EARLY DEVELOPMENT OF MAMMALS. 



the uterus. There are some experimental observations tending to prove the correct- 

 ness 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 externally 

 in the living ovum, the process manifests itself chiefly by the separating off of 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 remain- 

 ing 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. 



Maturation as a general process may be described as follows. For figures and 

 detailed accounts of the process in the mouse, see Chapter V. 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, the region of the ovum, which 

 contains most of the protoplasm and less of the yolk material. During the migra- 

 tion 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 dis- 

 appears. 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 surrounding protoplasm, causing the nucleus to become more or less 

 shriveled. The second step is the dissolution of the membrane of the nucleus so 

 that the nuclear contents are brought into direct contact and partly mixed 

 with the protoplasm of the cells. The third step, which in time more or less 

 accompanies the second, is the gathering of the chromatin of the nucleus into a 

 definite number of separate granules or chromosomes (tetrads'). These chromosomes 

 are always conspicuous and are larger than those formed during ordinary cell 

 division. Their number is also highly characteristic. As is now well known, there 

 appear during the process of indirect cell division a fairly definite number of chro- 

 mosomes, a number which is characteristic for each species. In numerous cases 

 it has been observed that the number of chromosomes in the maturing egg-cell is 

 exactly one half of that found during the ordinary cell divisions of the* same species.. 





