10 



HUMAN EMBKYOLOGY. 



Daughter centrosome 



Chromosomes 



dividing into 



equal parts 



PHASE OF HOMOTYPE MlTOSIS. 



Daughter centrosome 

 Chromosomes at ^- 

 pole of spindlex^ 



Achromatic / f. 

 spindle y. 



The chromatic substance is aggregated to form first a fine and afterwards a 

 thicker thread or spirem. At the same time, a spindle of achromatic fibrils appears 

 between the two daughter centrosomes, and the nuclear membrane disappears. 



As soon as the achromatic spindle is definitely established the chromatic 

 thread breaks up into a number of segments, the chromosomes, which arrange 

 themselves around the equator of the achromatic spindle. 



The chromosomes may be V-shaped, rod-like, cuboidal or spheroidal, and each 



may be a single structure, or it may consist of 

 two or four parts which are closely bound to- 

 gether. There is evidence which tends to sup- 

 port the belief that, whether the chromosome 

 appears to consist of one, two, or more seg- 

 ments, its constituent particles are derived 

 partly from the maternal and partly from the 

 paternal ancestor of the cell ; and it is believed 

 that the maternal and paternal portions undergo 

 similar division during the last three phases of 

 mitosis. In any case, whether the chromosomes 

 are single or compound structures, each becomes 



FIG. 6. SCHEMA OF ANIMAL CELL IN META- attached to, or very closely associated with, one 



of the fibrils of the achromatic spindle. 



At the end of the prophase the nucleus 

 as such, and the nucleolus, have entirely dis- 

 appeared, and the cell body contains, in their 

 place, two centrosomes, an achromatic spindle, 

 and the chromosomes. The centrosomes lie at 

 the opposite poles of the achromatic spindle 

 with the granules of the protoplasm grouped 

 radially around them, and the chromosomes are 

 grouped round the equator of the achromatic 

 spindle. 



The Metaphase. During the metaphase 

 each chromosome divides into two equal parts, 

 the rods or loops dividing longitudinally ; and 

 the division, in all cases, commences at the 

 point where the chromosome is in relation with 

 the fibrils of the achromatic spindle. 



The Anaphase. In the anaphase the halves 

 of the chromosomes, i.e. daughter chromosomes, 

 move towards the opposite poles of the achro- 

 matic spindle, and when they reach the vicinity 

 of the daughter centrosomes the anaphase ends 

 and the telophase begins. 



The Telophase. At the end of the anaphase, 

 or the commencement of the telophase, a con- 

 striction appears around the periphery of the 



FIG. S.-SCHBMA OF ANIMAL CELL AT END OF Cell > at the level of the e <l u ator of the achro- 

 TELOPHASE OF HOMOTYPE MITOSIS. The matic spindle. After its appearance the con- 

 cell has divided into two daughter cells, striction gradually deepens until the cell is 



Red and blue indicate the original paternal rnTYm lp4- p l v rKvidpfl info two halvpc; thp 

 and maternal derivatives LiUlllplt5 LolV U-lvlLlcU. lllLU LWU ilctlVco, uiit 



daughter cells, each of which contains the 

 typical number of chromosomes, and a portion of the achromatic spindle. 



The Resting Stage. During the resting stage, which lasts for a variable period, 

 a nucleus is formed in each daughter cell by the appearance of a nuclear membrane 

 around the chromosomes, as they repass first to the thread-like and then to the 

 granular form of chromatic substance, and by the reappearance of a nucleolus. 

 The cell increases in size also. 



The Period of Cell Life. The period of cell life varies, but in all cases it 

 ultimately ends in death ; for a time comes when cells no longer transmit to their 



; - 7 



FIG. 7. SCHEMA OF ANIMAL CELL AT END 

 OF ANAPHASE OF HOMOTYPE MITOSIS. 



Centrosome 

 Nucleus 



