STRUCTURE AND PHYSIOLOGY OF ANIMALS 



19 



two cells a narrow septum ; this is known as the cellulate (I., c.pl.~). 

 But a cell-plate is not of general occurrence in the division of the 

 animal 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 complicated 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 multicdlular 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. 6), 

 usually spherical in 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. 7, A). This consists, in essence, of 

 the throwing out of portions of the nucleus.' The latter 

 approaches the surface and divides, mitotically, into two parts 

 one coming to project on the surface and finally the projection being 

 completely separated off from the ovum as a rounded particle- 

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

 results in the throwing off of a second polar body ; and, after this 

 has been formed, the portion which remains in the ovum resumes 

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

 nudeus (B, $ pron.). The essential ultimate result of maturation 

 is the reduction of the number of chromosomes in the ovum by 

 one-half. 



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



c 2 



FIG. 6. 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- 

 matiii and a large nucleolus (ger- 

 minal spot). (From Balfour's Em- 

 bryology, after Hertwig.) 



