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. 



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

 same meaning in allot her 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 multicdltilar 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 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 yollc. 



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 in essence, of the throwing out of portions of the nucleus 

 Lhe latter approaches the surface and divides, mitotically, into two 

 parts one coming to project on the surface and finally becoming 

 completely separated off from the ovum as a rounded partu-le- 

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

 results m the throwing off of a second polar lody ; and, after this 

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

 its central position and forms what is termed the fcmal- mo- 

 nucleus ( 9 pro it.). 



In the proces.s of impregnation a very minute body, the mah 



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



I or wwm,and the nucleus whir-h it contains the male pro- 



Fir;. 4. Ovum of a Sea-Urchin, showing 

 the radially striated cell-membrane, 

 the protoplasm, containing y. ilk- 

 granules, the large nucleus (germinal 

 vesicle), with its network of chro- 

 matin and a large nucleolus (ger- 

 minal spot). (From Balfour's Eia- 

 bryology, after Hertwig.) 



