20 ZOOLOGY SECT. 



would appear as if they had an important share in controlling 

 the process, yet mitosis takes place during cell-division of the . 

 higher plants on the same general lines as in animals, though 

 centrosomes have rarely, if ever, been observed in plants higher 

 than the Mosses. 



A furrow which appears on the surface of the cell-protoplasm (Fig. 

 5, H, I), surrounding it in the form of a ring in a plane at right angles 

 to the long axis of the spindle, deepens gradually so as to give rise 

 to a cleft, eventually completely separating the substance of the 

 cell into two halves. Each of these halves encloses one of the 

 daughter-nuclei, and has assumed the character of a complete 

 daughter-cell. During this process there is sometimes distinguish- 

 able along the line corresponding to the division line between the 

 two cells a narrow septum composed of thickenings of the fibres of 

 the spindle ; this is known as the cell-plate (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. 



In many of the Protozoa the nucleus in certain conditions gives 

 off parts of its substance, which pass into its cytoplasm as indepen- 

 dent chromatin bodies, the chromidia, which may give rise to new 

 nuclei. The chromatin masses in chromidia are distinguished as 

 the cJiromidiosomes. 



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. 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 nudeoli (germinal spot or spots). 

 The ovum usually contains in addition to the protoplasm a 

 quantity of non-protoplasmic nutrient material or yolk. 



Before the process of impregnation 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 



