Chap. XXI] THE ORGANS OF GENERATION 461 



The cell-body is a mass of cytoplasm filled with fatty and 

 albuminous granules, and usually called the vitellus. 



The cell nucleus, or germinal vesicle, is a transparent, sharply 

 outlined nucleus, embedded in the vitellus. 



The nucleolus, or germinal spot, is a small, dark spot situated 

 in the fluid nucleus. 



The cell-body or cytoplasm contains the food material, and the 

 nucleus contains chromatin material. Chromatin is of special 

 interest because it is believed that it is through the chromatin 

 material that hereditary characteristics are transmitted. 



The spermatozoon. — The spermatozoon is the male generative 

 cell and its function is to fertilize the ovum and produce impregna- 

 tion. It is much smaller than the ovum, being only 5^ inc^ (0.1 

 mm.) in length. It consists of an elliptical head, a rod-shaped 

 middle piece, and a tail that gradually tapers. The head contains 

 nuclear material and chromatin. There is an active vibratory motion 

 of the tail which allows it quite free motion in the seminal fluid. 

 Because of this free motion the spermatozoa are able when depos- 

 ited in the vagina to travel upward into the uterus, and into 

 the tubes even against the current produced by the cilia of the 

 tubes. 



Site of impregnation. — It is thought that impregnation takes 

 place in the Fallopian tubes. When the Graafian follicle ruptures 

 and an ovum escapes into the abdominal cavity, the current pro- 

 duced by the cilia of the tubes is thought to draw it into the tube. 

 Once in the tube the peristaltic action of the tube and the action 

 of the cilia propel it slowly along to the uterus. If the ovum does 

 not become impregnated it passes into the uterus and is cast off 

 in the next menstrual flow. If, however, it comes in contact with 

 the spermatozoon in its passage through the tube, the spermato- 

 zoon enters the ovum and segmentation or the process of cell divi- 

 sion begins at once. 



Segmentation. — After the union of these two, the cell rapidly 

 divides into two, each of these two into other two, and so forth, 

 until we have a number of cells where formerly there was one. At 

 this stage the collection of cells is called the blastoderm. Grad- 

 ually these cells which constitute the blastoderm become arranged 

 in three layers, the outer called the ectoderm, an inner called the 

 entoderm, and a middle layer called the mesoderm. (See page 27.) 



