SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 159 



the chromatic material of each vesicle organizes in the form of a chromo- 

 some. The same condition is found in the nuclei of Fundulus (Richards 

 1917), Crepidula (Conklin 1902), and certain fish hybrids (Pinney 1918). 

 From this it is evident that the morphological identity of the chromo- 

 somes has not been lost between mitoses, although a very different type 

 of organization has been assumed. 



In Carex aquatilis Stout (1912) has found a peculiar condition. Here 

 the very small spherical chromosomes, which maintain a serial arrange- 

 ment, are visible in the resting state, and can be traced continuously 

 through all stages of the somatic and germ cell divisions with the excep- 

 tion of synizesis. 



The interpretations of Bonnevie (1908, 1911) and Dehorne (1911), 

 according to whom the chromosomes persist through the resting stage as 

 spirals or double spirals, have been mentioned in the description of 

 mitosis. 



Prochromosomes. Bodies known as prochromosomes have been 

 described in the nuclei of a number of plants : in Thalictrum, Calycanthus, 

 Campanula, Helleborus, Podophyllum, and Richardia by Overton (1905, 

 1909) ; in the Cruciferse by Laibach (1907) ; in Drosera and other forms by 

 Rosenberg (1909); in Acer platanoides by Darling (1914); in Musa by 

 Tischler (1910) ; and in a number of other forms. These prochromosomes 

 appear as small chromatic masses in the reticulum (Fig. 55, D), and 

 correspond approximately in number to the chromosomes of the species. 

 They are generally looked upon as portions of chromosomes which have 

 not undergone complete alveolation, and as centers about which the 

 chromosomes again condense at the next prophase. This interpretation is 

 in all probability a valid one in many of the described cases, but in others 

 the significance of such chromatic masses is questionable. In Crepis 

 virens de Smet (1914), in harmony with the conclusions of Miss Digby 

 (1914), finds them to be accumulations of material formed during the 

 resting stages. If such is the case they are to be regarded as karyosomes. 



Persistence of Parental Chromosome Groups After Fertilization. In 

 Chapter XII it will be shown that at fertilization there are brought 

 together two sets of chromosomes, one set from each parent; and that in 

 every nucleus of the resulting individual the chromosomes furnished by 

 the two parents are present together, all of them dividing at every mitosis. 

 When the chromosomes of the male parent are similar to those of the 

 female parent it is usually impossible to distinguish them in the nuclei 

 of the offspring. In a number of cases, however, such as Crepidula 

 (Conklin 1897, 1901), Cyclops (Hsecker 1895; Ruckert 1895), and Crypto- 

 branchus (Smith 1919) (Fig. 109), the two parental groups are distinguish- 

 able on the mitotic spindle, and often at other stages, through several 

 embryonal cell generations. It is in hybrids that this phenomenon is 

 shown most strikingly. In hybrid fishes obtained by crossing Fundulus 



