SEGMENTATION 337 



ical elements from both parents, and it is not surprising that 

 the child should resemble both, anatomically and otherwise. 



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

 pregnated sexual cell discharged from the female ovary. It is 

 also used to signify the fertilized cell, and is in fact often ap- 

 plied without much precision to the product of conception 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 pas- 

 sage it becomes covered with a coating of albuminous material. 

 This layer is probably impervious to spermatozoa which fact 

 may account for the practical universality 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 thick- 

 ened mucous membrane of that organ in a way to be noted pres- 

 ently. Here it remains until expelled during parturition. 



Segmentation. As soon as union of male and female pro- 

 nuclei has taken place, cleavage of the ovum begins. The nu- 

 cleus (segmentation nucleus) and protoplasm divide by karyo- 

 kinesis 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 perivi- 

 telline membrane. As division proceeds, cells arrange them- 

 selves 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 covering 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 large hollow sphere. The space 

 thus left between the two kinds of cells is called the segmentation 

 cavity. Soon the surrounding cells become attenuated (Rauber's 

 cells) and disappear. Their place, as a surrounding envelope, 



