SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 163 



Beneden, and Strasburger that the number of chromosomes in any given 

 species is relatively constant. It was largely upon this fact that the 

 theory of chromosome individuality was originally based: the fact that 

 the number of chromosomes appearing at every mitosis is almost invari- 

 ably the same was taken to mean that the structural identity of the 

 chromosomes is never lost. Certain observers (Fick 1905, 1909) have 

 held that the apparent constancy in number is not due to a structural 

 continuity or individuality of any sort, but rather to the fact that the 

 successive nuclei have a relatively uniform amount of nuclear material, 

 the chromosomes "crystallizing out" of this material in each prophase 

 and going into solution at the close of mitosis. This idea was especially 

 developed by Delia Valle (1909, 1912a&), who described the formation 

 of chromosomes by the aggregation of fluid crystals during the prophase. 

 These chromosomes he held to be in no sense morphologically continuous 

 individuals, but only temporary chromatic accumulations which are in- 

 constant in number and lose their identity in the telophase. Delia Valle's 

 interpretation of chromosome formation has been criticized by a number 

 of writers and his position shown to be untenable by Montgomery (1910), 

 McClung (1917), and Parmenter (1919). 



Some of the experiments on echinoderm eggs with which Boveri (1895, 

 1902, 1903, 19046, 1905, 1907) and others supported the theory of chromo- 

 some individuality may be briefly reviewed. 



Boveri found that if the number of chromosomes is increased or de- 

 creased by artificial means the altered number appears at every mitosis 

 thereafter, (a) An enucleate egg fragment may be entered by a sperma- 

 tozoon, and may then develop into a larva with half the normal number 

 of chromosomes in every cell. (&) In another experiment the unfertilized 

 egg of a sea urchin was caused to undergo division by artificial means, 

 after which a spermatozoon was allowed to enter one of the blastomeres 

 (daughter cells) . A larva resulted in which one-half of the cells had regu- 

 larly 18 chromosomes (half the normal number) while the other half had 

 the normal 36. (c) Two spermatozoa occasionally fertilized one egg: 

 the cells of the resulting larvae had 54 chromosomes, the triploid number. 

 Abnormal mitotic figures were often formed in such dispermic eggs, 

 bringing about an irregular distribution of the chromosomes. For ex- 

 ample, a quadripolar spindle was produced, separating the 54 split chromo- 

 somes (108 daughter chromosomes) into four groups, with 18, 22, 32, and 

 36 chromosomes respectively (Fig. 127 bis). The resulting abnormal 

 larva ("pluteus") showed these four chromosome numbers in the cells 

 of four different regions of its body. Boveri (1914) later suggested that 

 malignant tumors might be due to such abnormal chromosome distri- 

 bution, (d) The number of chromosomes was doubled by shaking the 

 eggs while the chromosomes were split during the early stages of cell- 

 division. In this manner larvae wore produced with 72 chromosomes, the 



