GERM-CELL HISTORY IN THE BROOK LAMPREY 87 



among vertebrates. If it occupy such a position, it might very 

 easily escape observation. In plants no accessory chromosome 

 has been found, except in Salamonia biflora in which Cardiff 

 ('06) describes one, but his interpretation has been doubted by 

 Strasburger and others. It is in insects that the accessory chro- 

 mosome has been studied with most care, and in this group it 

 has also been found that the secondary sexual characters appar- 

 ently develop independently of the sex glands. It has been 

 shown by Kellogg ('04), Meisenheimer ('09), Kopec ('11), and 

 Steche ('12) that in moths, at least, the presence of a particular 

 germ gland in the animal is not responsible for the development 

 of the secondary sexual characters. In this case the primary and 

 secondary sexual characters seem to develop in consequence of the 

 presence in the developing embryo of a common factor or set of 

 factors which may be located in sex chromosomes. 



Many of the bodies that have been described as accessory or 

 sex chromosomes are probably something else. Our knowledge 

 concerning many of the cytoplasmic bodies in the cell is very 

 limited, but it is known that some of them may occur among the 

 chromosomes during mitosis. It will be recalled that Wilson ('13) 

 in his work on Pentatoma warned against mistaking a so-called 

 chromatoid body in certain cells for a sex chromosome. Wod- 

 sadelek ('14) found a similar body in sex cells of the horse, and 

 Bachhuber ('16) found it in the rabbit. There also seems to be a 

 certain relation between the nucleolus of the cell and the acces- 

 sory chromosome in certain cases. Goldsmith ('16) thinks he 

 has found evidence in Pselliodes cinctus that the nucleolus is 

 composed of both chromatic and achromatic material. The 

 achromatic material he thinks is linin, or closely related to it 

 in composition, while the chromatic part constitutes the sex 

 chromosome. 



It must be admitted, however, that an accessory chromosome 

 is undoubtedly present in the cells of a great number of forms, 

 and that it may function as a sex determiner, at least in the 

 absence of other factors. There is, however, good reason for 

 believing that it forms only one link in a series of events that 

 precede the development of sex. This conclusion has also been 



