908 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



overlap each other in time. The first act is characterized by the appear- 

 ance of two centrosomes, each with its astral rays, in place of the one 

 already existing (Fig. 311, c). The two take up positions on opposite sides of 

 the nucleus (Fig. 311, D) and await the time when they can exert their specific 

 function. We have spoken of the difference of opinion regarding the origin 

 of the original centrosome of fertilization. The origin of the two centrosomes 

 present in segmentation has likewise been disputed. The question is of consid- 

 erable theoretical interest in connection with the problem of the physical basis 

 of inheritance. Certain observers have claimed that the centrosomes have a 

 double origin, one being derived from the male and one from the female germ- 

 cell. Upon this theory sexuality is shown by the cytoplasmic centrosomes as 

 well as by the nuclear chromosomes, and the inference is possible that cytoplasm, 

 as well as nucleus, transmits hereditary qualities. The observations of Boveri, 

 Wilson, and others refute this claim by showing that the two centrosomes arise 

 by a splitting of the original centrosome, which is derived from the middle- 

 piece of the spermatozoon. They are, therefore, not male and female, and 

 cannot be regarded as bearers of inherited characteristics. These observa- 

 tions not only allow, but tend to strengthen, the prevailing view of the 

 exclusive hereditary role of the nucleus. (See below under Heredity, 

 p. 931). 



The second act of segmentation is more complicated than the first, and con- 

 sists of a halving of the nucleus. The nuclear membrane gradually disap- 

 pears. The achromatic network resolves itself into long cytoplasmic filaments 

 arranged in the form of a spindle, and meeting at the two centrosomes (Fig. 

 312, A). The spindle, centrosomes, and asters form the body known as the am- 

 phiaster. The chromatic substance becomes changed into the definite rod-like 

 chromosomes which are collected in the equatorial zone of the spindle and con- 

 stitute the equatorial plate (Fig. 312, B). From the observations of Van 

 Beneden, Riickert, 1 Zoja, 2 and others, it seems probable that the male and 

 the female chromosomes do not fuse together, but remain distinct from each 

 other, perhaps throughout all the tissue-cells. Each chromosome proceeds 

 to split lengthwise, and the two halves are drawn toward the two centro- 

 somes, being mechanically pulled, it is commonly believed, by contraction 

 of the spindle-filaments, assisted by the astral rays (Fig. 312, c). The two 

 halves of the amphiaster, each with its centrosome, are, in fact, commonly 

 believed to be composed of contractile cytoplasm and to be organs possessing 

 the definite function of separating the two halves of the nucleus in karyokinesis. 

 The evidence for this view is not wholly satisfactory. In the process of divis- 

 ion each nuclear half obtains half of the original male and half of the original 

 female chromatin, and hence contains inherited potentialities of both parents. 

 After division each half gradually assumes the structure of a typical resting 

 nucleus with its accompanying aster. 



The third act of segmentation consists of a simple division of the cytoplasm 



1 J. Riickert: Archiv fur mikroskopische Anatomie, xlv., 1895. 



2 R. Zoja: Anatomischer Anzeiger, xi., 1896. 



