GROWTH, REGENERATION AND REPRODUCTION 1111 



conjugation and which undoubtedly is a prototype of the interaction 

 of the germ-cells of the multicellular forms. Conjugation is essen- 

 tially a union of the nuclei of the conjugating cells, although in unicellu- 

 lar plants the cell-bodies -are fused as well, while in the infusoria this 

 union is only temporary. Maupas^ believes that this process invari- 

 ably follows a long period of multiplication by cell-division and may 

 be compared to the attainment of sexual maturity of the higher 

 animals. According to Biitschli, its purpose is to prevent senile 

 retrogressive changes and to instil new vigor into the descendents. 

 In the infusoria, Wilson^ recognizes the following changes: To 

 begin with, each cell possesses two kinds of nuclei, namely, a large 

 macronucleus and one or several micronuclei. As soon as the cells 

 have become applied, the former degenerates and disappears. In 

 consummating this process, the micronucleus divides twice to form four 

 spindle-shaped bodies. While three of these degenerate, the fourth 

 splits into smaller masses. These micronuclei are then exchanged, 

 one from A passing into B and one from B into A . Very soon after 

 these cells have again separated each pair of nuclear masses unite into 

 one. This single micronucleus then divides three times to form eight; 

 while the cell meanwhile splits into four parts, two nuclei being 

 apportioned to each daughter-cell. One of the latter enlarges to form 

 the macronucleus, while the other continues as the micronucleus. 



Indirect Cell-division or Mitosis. — By far the greatest number of 

 animal and vegetable cells multiply by the process of mitosis or 

 karyokinesis, which differs from amitosis chiefly in the fact that the 

 nucleus undergoes a number of very characteristic changes. In order 

 to be able to follow these more conveniently, they may be divided into 

 the following phases: 



(a) Prophases, during which the division is initiated. 



(6) Metaphase, during which the nucleus undergoes its most important change. 



(c) Anaphases, during which the nuclear material is arranged in a peculiar 

 manner, preparatory to the 



(d) Telophases, during which the active cell divides, giving rise to the daughter- 

 cells. 



During the prophase the chroniatine substance of the nucleus acquires a greater 

 power of staining, loses its net-like character and is eventually resolved into a 

 definite number of separate bodies possessing intense staining qualities. These 

 so-called chromosomes are generally rod-shaped, straight or curved, but may also 

 be spherical, ovoidal or ring-like. They arise in consequence of the transverse 

 division of the spireme-thread into which the nuclear substance first resolves it- 

 self. In the place previously occupied by the nucleus, the cytoplasm assumes a 

 radiate appearance, giving rise to a star or aster. In the center of each aster lies 

 a centrosome, while in between them is a spindle of fine fibers, known as the 

 achromatic spindle. The otiromosomes arrange themselves in a plane at the 

 equator of the spindle. 



The metaphase is characterized by a lengthwise splitting of the chromosomes 

 into equal halves, thus initiating the actual division of the cell. During the ana- 

 phase these daughter-chromosomes move toward the opposite poles of the spindle 



1 Arch, de Zoologie, Sec. II, vii, 1889. 



2 The Cell in Development and Inheritance, Macmillan, 1919. 



