18 



ZOOLOGY 



SECT. 



of these halves encloses one of the daughter-nuclei and has 

 assumed the character of a complete daughter-cell. In some 

 instances the division of the nucleus is direct or amitotic, the 

 nucleus simply becoming separated into two equal parts, without 

 disappearance of the nuclear membrane, and without any compli- 

 cated re-arrangement of the chromatin. 



3. THE OVUM : MATURATION, IMPREGNATION, AND SEGMENTATION : 

 THE GERMINAL LAYERS. 



Amoeba is simply an independent animal cell; or, to express the 

 same meaning in another way, is a unicellular animal, and as such 

 it is a member of the phylum of the Protozoa or unicellular 

 animals. All the rest of the animal kingdom, forming the 

 division Metazoa, are multicellular in the fully developed condition ; 



but each of these multicellular 

 animals or Metazoa originates from 

 a single cell the ovum. The 

 ovum is a typical cell (Fig. 4), 

 usually spherical jn shape, with 

 one or more enclosing membranes,, 

 with cell protoplasm enclosing a 

 nucleus (germinal vesicle) in which 

 are contained one or more rounded 

 masses of chromatin (germinal spot 

 or spots). The ovum may contain 

 in addition to the protoplasm a 

 quantity of non-protoplasmic nu- 

 trient material or yolk. 



Before the process of impregna- 

 tion or fertilisation which gives the 

 impulse to development, the ovum 

 undergoes a change which is termed 

 maturation (Fig. 5, A). This con- 

 sists m essence, of the throwing out of portions of the nucleus. 

 I he latter approaches the surface and divides, mitotically, into two 

 parts -one coming to project o % the surface and finally becoming- 

 completely separated off from the ovum as a rounded particle^ 

 the first polar lody (pol). A second division of the nucleus 

 results in the throwing off ol I polar lody; and, after this 



has been formed, the portion which n>mains in the ovum resumes 

 its central position and fomw what is termed the female pro- 

 nucleus (<j> pr (>n.}. 



In the process of impregnation a very minute body, the mak 

 cell werm-cell, or sperm, penetrates into the interior of the female 

 cell or Gtwm,and the nucleus ^hk-li it contains the male pro- 



FIG. 4. Ovum of a Sea-Urchin, showing 

 the radially striated cell-membrane, 

 the protoplasm, containing yolk^ 

 granules, the large nucleus (germinal 

 vesicle), with its network of chro- 

 matin and a large nucleolus (ger- 

 minal spot). (From Balfour's E,n- 

 bryology, after Hertwig.) 



