n6 COMPARATIVE ANATOMY 



spindle; they determine, by the positions which they take up, 

 the direction of the spindle, and consequently 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 pre- 

 cociously 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 segmentation of the 

 spireme. It is clear, therefore, that the centrosome and the 

 chromosomes act independently of one another in the process 

 of cell-division. 



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. 



The generative organs of the frog are recognisable at a very 

 early period in its life history. In a tadpole of about 10 mm. 

 length an accumulation of primitive germ-cells may be seen 

 lying between the two folds of the peritoneal epithelium forming 

 the origin of the mesentery in the middle third of the pleuro- 

 peritoneal cavity. In a microscopical preparation the cytoplasm 

 of the primitive germ-cells is seen to be filled with platelets of 

 reserve material or yolk, the presence of which makes these 

 cells easily distinguishable from the neighbouring epithelial and 

 connective tissues. The rudiment of the generative organs is 

 at first unpaired, but it soon becomes a flat band and is divided 

 lengthwise into right and left halves which, as growth proceeds, 

 project into the pleuro-peritoneal cavity as two ridges lying 

 right and left of the mesentery, and covered in below by the 

 flattened peritoneal epithelium. The primitive germ-cells 

 increase in size, seemingly at the expense of the reserve 

 material stored up within them, for by the time that they 

 have attained their full size the yolk platelets have disap- 

 peared. The primitive germ-cells then multiply by a number 

 of successive divisions, in which the nuclear mitoses are 



