GENERAL SKETCH 135 



('90) showed that the law of numerical equality of the paternal and 

 maternal chromosomes held good for other groups of animals, being 

 in the sea-urchin Echinus 9, in the worm Sagitta 9, in the medusa 

 Tiara 14, .and in the mollusk PtcrotracJiea 16 from each sex. Similar 

 results were obtained in other animals and in plants, as first shown by 

 Guignard in the lily ('91), where each sex contributes 12 chromosomes. 

 In the onion the number is 8 (Strasburger) ; in the annelid Ophryo- 

 trocha it is only 2 from each sex (Korschelt). In all these cases the 

 number contributed by each is onc-Jialf the number characteristic of the 

 body-cells. The union of two germ-cells thus restores , the normal 

 number, and thus we find the explanation of the remarkable fact 

 commented on at p. 48 that tJie number of chromosomes in sexually 

 produced organisms is always even. 1 



These remarkable facts demonstrate the two germ-nuclei to be in 

 a morphological sense precisely equivalent, and they not only lend 

 very strong support to Hertwig's identification of the nucleus as the 

 bearer of hereditary qualities, but indicate further that these qualities 

 must be carried by the chromosomes ; for their precise equivalence in 

 number, shape, and size is the physical correlative of the fact that 

 the two sexes play, on the whole, equal parts in hereditary transmis- 

 sion. And thus we are finally led to the view that chromatin is the 

 physical basis of inheritance, and that the smallest visible units 

 of structure by which inheritance is effected are to be sought in the 

 chromatin-granules or chromomeres. 



2. The Centrosome in Fertilization 



The origin of the centrosomes and of the amphiaster, by means of 

 which the paternal and maternal chromosomes are distributed and 

 the egg divides, is still in some measure a matter of dispute. In a 

 large number of cases, however, it is certainly known that the egg-cen- 

 trosome disappears before or during fertilisation and its place is taken 

 by a new centrosome ivhich is introduced by the spermatozoon and 

 divides into tzvo to form the cleavage-ampJiiaster. This has been 

 conclusively demonstrated in several forms (various echinoderms, 

 annelids, nematodes, tunicates, mollusks, and vertebrates) and estab- 

 lished with a high degree of probability in many others (insects, crus- 

 tacea). In every accurately known case, moreover, the centrosome 

 has been traced to the middle-piece of the spermatozoon ; e.g. in 

 sea-urchins (Hertwig, Boveri, Wilson, Mathews, Hill), in the axolotl 

 (Pick), in the tunicate Phallusia (Hill), probably in the earthworm, 



1 Cf. p, 154. 



