THE MITOTIC CYCLE 



Even if further work should show that adult nuclei with equal num- 

 bers of chromosomes always contain a constant amount of DNA, this 

 probably does not generally apply to the early embryo. The nucleus 

 of the fertilized egg is larger than those of the blastomeres (p 63) ; in the 

 sea-urchin the DNA content of the nuclei decreases during cleavage, 

 but is not apparently directly proportional to nuclear size. The DNA 

 content of embryonic echinoderm nuclei has been studied by 

 Bracket,*^ Schmidt et alii,^* Villee et alii,^^ and by Mazia.^" The 

 latter author has clearly summarized the results in this field which have 

 so far been obtained. The DNA content of the echinoderm embryo 

 increases steadily, but at a rate much slower than that of cell multiplica- 

 tion, so that in Arbacia at the tenth hour, the nuclei contain only 

 one twentieth to one thirtieth of the DNA of the fertilized egg. The 

 frequency of cell division then decreases slightly and so by the fortieth 

 hour this fraction has increased to about one tenth. It would be very 

 desirable to follow the subsequent course of events towards the adult 

 condition in these respects. 



Alfert^^ finds that the range of relative DNA content of the nucleus 

 of the mouse egg apparently remains constant during early cleavages, 

 and approximate to that of the primary oocytes. 



Until the varying relationships of the DNA content of nuclei with 

 age and size are better understood, it may be unsafe, as Davidson and 

 Leslie ^2 have done, to base work on the growth and development of 

 tissues on an assumed fixity of this quantity. The evidence for the syn- 

 thesis of DNA between mitotic periods is increasing. This subject is 

 developing very rapidly, and it may be that fresh observations will 

 have appeared before the publication of this book. 



THE nucleoli 



In general, the nucleoli are the most obvious feature of resting nuclei, 

 though sometimes in early stages of development they are absent 

 (LuDFORD,^^ Brachet^*). In the history of microscopical study, the 

 first trace of the nucleolus is to be found in 1 781, in a book by Fontana,^^ 

 mainly concerned with the viper and its venom. In one section of this 

 work are described a number of rather miscellaneous observations; 

 within an epithelial cell or 'vesicule' from the slime of an eel, he tells us, 

 'on observe un corps oviforme, ay ant une tache dans son milieu'. The 

 nucleus, Fontana's egg-shaped body, was first described by Leeuwen- 

 HOEK in 1702 (Baker^^). 



Distinction between true nucleoli and chromosome-nucleoli 



Many papers in all have been written on the nucleolus; among 

 reviews on this subject may be mentioned those of Montgomery,^' 



38 



