22 



THE ANIMAL CELL. 



[CH. II. 



The Ovum. 



The ovary is an organ which produces ova. 



An ovum is a simple animal cell ; its parts are seen in the next 

 diagram. 



It is enclosed in a membrane called the zona pellucidaor vitelline 

 membrane. The body of the cell is composed of protoplasm loaded 



--- Nucleus or germinal vesicle. 

 -- Nucleolus or germinal spot. 



Space left by retraction of 

 protoplasm. 



. . Protoplasm containing yolk 

 spherules. 



Vitelline membrane. 



Fig. 26. Representation of a human ovum. (Cadiat.) 



with granules of food material, and often called the yolk or vitellus. 

 The nucleus and nucleolus are sometimes still called by their old 

 names, germinal vesicle and r/erminal spot respectively. 



The formation of ova will form the subject of a chapter later 

 on, but it is convenient here at the outset to state briefly one or 

 two facts, and introduce to the student a few terms which we 

 shall have to employ frequently in the intervening chapters. 



The ovum first discharges from its interior a portion of its 

 nucleus, which forms two little globules upon it called the polar 

 globules. 



Fertilisation then occurs ; that is to say, the head or nucleus 

 of a male cell called a spermatozoon penetrates into the ovum, 

 and becomes fused with the remains of the female nucleus. 



Cell division or segmentation then begins, and the early stages 

 are represented in the next figure. 



Fluid discharged from the cells accumulates within the interior 

 of the mulberry mass seen in fig. 27 d, and later, if a section is 

 cut through it, the cells will be found arranged in three layers. 



The outermost layer is called the epiblast. 



The middle layer is called the mesoblast. 



The innermost layer is called the hypoblast. 



From these three layers the growth of the rest of the body 

 occurs, nutritive material being derived from the mother in 

 mammals by means of an organ called the placenta. 



