CH. II.] THE OVUM 21 



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, 



FIG. 27. 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.) 



nutritive material being derived from the mother in mammals by 

 means of an organ called the placenta. 



The epiblast, the outermost layer of the embryo, forms the epi- 

 dermis, the outermost layer of the adult. It also forms the nervous 

 system. 



The liypollast, 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 systems. 



