2l8 ANIMALS 



in the other. In the simpler type the first evidence that di- 

 vision is about to take place is seen in a slight elongation of the 

 nucleus. This proceeds until the nucleus assumes the shape of 

 a dumb-bell. The two halves continue to draw apart until 

 only a slender strand connects them and this finally breaks. 

 As the division of the nucleus proceeds the body of the cell also 

 elongates. The pseudopodia are formed only at the two ends. 

 The cell becomes constricted in the equatorial plane and this 

 cuts deeper into the cell until the latter is finally cut into 

 two approximately equal parts. The two daughter nuclei have 

 by this time assumed the normal rounded form and there are 

 then two amoebae. In the division the contractile vacuole 

 remains in one of the daughter cells, but before division is 

 complete a new vacuole has been formed in the other one. 



497. In other cases, division comes about through a compli- 

 cated process known as mitosis or karyokinesis. This process 

 is described below. No significance is known to attach to the 

 difference in method. The results are apparently the same. 



498. In many Protozoa, another interesting phenomenon 

 has been observed which should be mentioned here, although 

 it has not been observed in amoeba. This is the phenomenon 

 of conjugation. Two similar animals unite, either partially 

 and temporarily or else completely, so as to form a single cell. 

 In the latter case the two nuclei fuse into one. When the 

 union is only temporary the nuclei of both cells divide and a part 

 of the nuclear matter from each cell is transferred to the other 

 cell, where it unites with the nucleus of that cell. By either of 

 these processes cells are formed with nuclei composed of ma- 

 terial derived in equal parts from two individuals. The 

 details of this process will be discussed more fully in Part III. 

 Its significance will be better understood when compared with 

 the sexual method of reproduction of the metazoa. 



499. Hydra reproduces by budding and by development of 

 eggs. Budding is a process found among other metazoa as 



