REPRODUCTION 

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the male element, and they coalesce to become the segmenta- 

 tion nucleus. Impregnation has now taken place. The seg- 

 mentation nucleus represents a new being. It contains ana- 

 tomical elements from both parents, and it is not surprising 

 that the child should resemble both, anatomically and other- 

 wise. 



The term "ovum" has so far been used to signify the un- 

 impregnated sexual cell discharged from the female ovary. 

 It is also used to signify the fertilized cell, and is in fact 

 often applied without much precision to the product of con- 

 ception at almost any stage of its intrauterine development. 



The fertilized ovum is carried through the tube to the 

 uterus, arriving there some seven days after its fecundation. 

 In its passage it becomes covered with a coating of albu- 

 minous material. This layer is probably impervious to sper- 

 matozoa which fact may account for the practical univer- 

 sality of fecundation in the outer part of the tube, if at all. 

 The coating corresponds to the white of an egg, in that it 

 penetrates the perivitelline membrane and furnishes nutritive 

 material to the vitellus. On reaching the uterus the ovum 

 becomes attached to and covered by the thickened mucous 

 membrane of that organ in a way to be noted presently. 

 Here it remains until expelled during parturition. 



Segmentation. As soon as union of male and female 

 pronuclei has taken place, cleavage of the ovum begins. The, 

 nucleus (segmentation nucleus) and protoplasm divide kary- 

 okineses to form two nearly similar cells. These two divide 

 into four, these four into eight and so on, till a large number 

 of cells occupy the vitelline space and are all surrounded by 

 the perivitelline membrane. As division proceeds, cells ar- 

 range themselves around others, so that the former occupy 

 the circumference and the latter the center of the vitelline 

 cavity. Later, while the outer cells constitute a layer cover- 

 ing the entire inner surface of the perivitelline membrane, the 

 inner cells group to form a mass which is in contact with the 

 outer layer at one point only like a ball lying in a relatively 



