ii6 COMPARATIVE ANATOMY 



grows to the size of the original mother-nucleus, the chromatin 

 sharing in the growth of the whole nucleus ; and when the 

 normal size is attained the prophase is again entered into, and 

 a fresh mitosis with a new division of the cell takes place. 

 The behaviour of the centrosome demands some attention. 

 This minute body seems to lead the way in cell-division. The 

 two halves into which it divides form the centres of the asters 

 and the poles of the spindle ; they determine, by the positions 

 which they take up, the direction of the spindle, and conse- 

 quently the plane in which the cell-division will take place. 

 It is evident that this plane must always be at right angles to 

 the long axis of the spindle. The centrosomata sometimes 

 divide very precociously during the telophase or even during 

 the anaphase of mitosis, and the two products then remain in 

 each daughter-cell throughout the resting stage. On the other 

 hand, there are cases in which the division of the centrosome 

 is retarded until after the formation and even after the segmen- 

 tation of the spireme. We cannot, therefore, regard the 

 centrosome as a "dynamic centre," in the sense of its giving 

 an impulse to cell-division ; it seems rather to exercise a 

 directing influence, inasmuch as it determines the direction of 

 the long axis of the achromatic figure. 



The details of mitosis differ in different animals and plants, 

 but the end result in tissue-cells is always the same, the 

 chromatin of the mother-nucleus is divided into two equal and 

 like halves which are distributed to the daughter-nuclei. The 

 only real exceptions occur in abnormal and pathological cases, 

 such as cancer-cells, which need not detain us here. But, 

 whilst the equal division of the chromatin is the rule for tissue- 

 cells, the case is different for germ-cells. 



In a tadpole of the frog of about lo mm. in length, the 

 primordia of the generative organs are first found as a pair of 

 ridge-like thickenings of the peritoneal epithelium lying along 

 the dorsal side of the coslom close to the mesentery. Each 

 genital ridge is simply a thickening or modification of the 

 peritoneal epithelium, whose cells, elsewhere flattened, here 

 become cubical or columnar. As age advances, the genital 

 ridges become thickened, partly by the division of the cubical 

 germ-cells, which come to form a layer several cells thick, 

 partly by the ingrowth of connective tissue amongst the 

 developing cells. At this period there is no distinction of 



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