BE PROD UCTION. 



469 



/ 



m< 



tozoon divides into the two segmentation-centrosomes, the latter hence being 

 exclusively of male origin. Still 

 others believe that the sperm- 

 centrosome disappears, its place 

 being taken by two new ccntro- 

 somes derived from the cyto- 

 plasm of the egg. The evi- 

 dence available at present does 

 not allow a decision to be made 

 between these two latter views. 1 

 According to both of them, 

 however, the cleavage-eentro- 

 somes are not male and female, 

 and cannot be regarded as 

 bearers of inherited character- 

 istics. These observations not 

 only allow, but tend to 

 strengthen, the prevailing view 

 of the exclusive hereditary role 

 of the nucleus. (See below 

 under Heredity, p. 493.) 



The second act of segmenta- 

 tion is more complicated than 

 the first, and consists of a halv- 

 ing of the nucleus. The nuclear 

 membrane gradually disappears. 

 The achromatic network resolves 

 itself into long cytoplasmic fila- 

 ments arranged in the form of a 

 spindle, and meeting at the two 

 ccntrosomes (Fig. 225, a). The 

 spindle, ccntrosomes, and asters 

 form the body known as the 

 amphiaster. The chromatic sub- 

 stance becomes changed into the 

 definite rod-like; chromosomes, 



which are collected in the equa- tag as delicate filaments extending in the f< 



. , « , • n l from poll' tn polr. 



tonal zone ot the spindle, ana B , Each chromosome has become split into two. and the 



constitute the equatorial plate, latter are being pulled toward the poles. 



* * c. The divergence oi the chromosomes has ceased and 



Each chromosome proceeds to the Latter are becoming converted Into vesicular masses 



split lengthwise and the two , "' si '''' the centrosomes. The spindle is becoming resolved 



'' & * Into ordinary cytoplasm, 

 halves move toward the t\v<> 



ccntrosomes (Fig. 22o, b). The cause of this movement is not known. The 



'For a critical review of iliis and oilier problems in fertilization and segmentation Bee E. 

 B. Wilson: The Cell in Development and Inheritance, 1900, 2d ed., New York. 



Fig 225.— Stages in the segmentation of the egg (after 

 Wilson). The drawings were made from sections of eggs 

 of the sea-urchin, Toxopneuxtes variegatus, kg. 



a. The nuclear membrane has disappeared within the 

 nucleus a distinction between the chromatic and the achro- 

 matic substance has been made, the former existing as 

 clearly defined chromosomes aggregated in the centre to 

 form the equatorial plate, the achromatic substance exist 



nn of a spindle 



