CH. IL] 



THE OVUM 



19 



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. 



Nucleus or germinal vesicle. 

 ~,v^ Nucleolus or germinal spot. 



Space left by retraction of 

 protoplasm. 



Protoplasm containing yolk 

 spherules. 



Zona pellucida. 



FIG. 23. Representation of a human ovum. (Cadiat.) 



Cell division or segmentation then begins, and the early stages 

 are represented in the next figure. 



FIG. 24. Diagram of an ovum (a) undergoing segmentation. In (b) it has divided into two, in (c) into 

 four; and in (d) the process has resulted in the production of the so-called "mulberry-mass." 

 (Frey.) 



Fluid discharged from the cells accumulates within the interior 

 of the mulberry mass seen in fig. 24 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. 



The epiblast forms the epidermis, the outermost layer of the 

 adult. It also forms the nervous/system. 



The hypoblast, the innermost layer of the embryo, forms the 

 lining epithelium of the alimentary (except that of the mouth and 

 anus, which are involutions from the epiblast) and respiratory tracts, 

 that is, the innermost layer Of the adult. It also forms the cellular 

 elements in the large digestive glands, such as the liver and pancreas, 

 which are originally, like the lungs, outgrowths from the primitive 

 digestive tube. 



The mesoblast forms the remainder, that is, the great bulk of the 

 body, including the muscular, osseous, and other connective tissues ; 

 the circulatory and urino-genital system* 



