EXISTING ORGANISMS— PHYSIOLOGY 101 



magnet, with the two centrosomes representing the poles and the 

 fibers lines of force. For obvious reasons the radiating fibers are 

 called astral and the others spindle fibers. During the formation 

 of the spindle the chromatin becomes condensed into a compact 

 thread called the spireme, the nuclear membrane breaks down, and 

 all nuclear structures except the spireme become a part of the 

 cytoplasm. The spireme breaks up into a number of parts called 

 chromosomes, and these bodies migrate into the equator of the 

 spindle. The number of chromosomes varies in different species; 

 there are four in some worms, forty-eight in man, and even more 

 in some animals. This much of the process is called the prophase. 

 During the next stage, the metaphase, each chromosome splits 

 longitudinally into apparently equal halves. Following this the 

 halves migrate toward the nearest aster in the anaphase, and the 

 final telophase includes the constriction and splitting of the cyto- 

 plasm into two parts, each including one centrosome and one set 

 of chromosomes, and the reconstruction of the nuclei as in the 

 original parent cell. 



Reproduction of Individuals. In its simplest state the repro- 

 duction of individuals is no more than cell reproduction. In the 

 lower Mctazoa, however, it passes beyond this simplicity since 

 it must involve many specialized cells, but even here a process of 

 fission or budding is common which is little more than the repro- 

 duction of numerous cells of the parent to form a new, similar 

 individual. In all Metazoan phyla, an additional specialization 

 occurs, cither associated with the process of fission or budding or 

 the sole method of reproduction. In it the reproductive power 

 of certain cells of the body is emphasized to the extent that they 

 are developed solely for the purpose of producing new individuals, 

 so that the complete differentiation of the species from a single 

 cell occurs anew with every generation. To this is added gametic 

 or sexual reproduction, involving the union of two independent 

 germ cells. The modification of cells accompanying sexual repro- 

 duction is a further addition to the intricacies of mitosis, but it has 

 such an important bearing on other phases of our subject that it 

 will be taken up in connection with the laws of genetics. 



Accessory Reproductive Functions. With the establishment of 

 sexual reproduction the germ cells, or gametes, are formed from 

 ectoderm or endoderm in diploblastic animals, but in the triplo- 

 blasts they are consistently mesodermal as far as can be determined. 



