Section IV 



THE NUCLEUS AND CYTOPLASM 

 DEVELOPMENT 



IN 



CHAPTER 1 



The Role of Nucleus and Cytoplasm 



G. FANKHAUSER 



INTRODUCTION 



One of the important tasks of experimental 

 embryology is to trace the origin of the or- 

 ganization of the embryo back to fertiliza- 

 tion, the earliest stage of development at 

 which the egg becomes available for exten- 

 sive experimentation. It is at this stage that 

 the most favorable conditions obtain for an 

 analysis of the roles played in early develop- 

 ment by the tv^^o main components of the egg 

 cell, the cytoplasm and nucleus, one de- 

 ceptively simple in its visible structure, the 

 other known to be highly complex and or- 

 derly in its organization. Although the first 

 attempts to investigate this problem date 

 back to the earliest days of developmental 

 mechanics, our knowledge, even now, is far 

 from complete (cf. Needham, '42, pp. 351- 

 364). Some of the old experiments are in 

 need of completion or re-examination with 

 the aid of new, or at least more refined, 

 techniques. The manifold possibilities of at- 

 tack and the main results obtained thus far 

 will be outlined briefly in this chapter. 



While it is possible in any one experi- 

 ment to concentrate the attack only on the 

 nucleus or on the cytoplasm, interrelations 

 between the two components are ever pres- 

 ent (cf. Fankhauser, '52). The existence of 

 complex interactions from the very begin- 

 ning of development is evidenced by the study 

 of the processes of fertilization itself and of 

 its various experimental modifications. Wit- 

 ness the frequent disengagement of the nu- 

 clear and cytoplasmic (centrosomal) cycles 

 of mitosis under experimental conditions, 

 producing abnormal types of mitosis; the 



peculiar behavior of the spermatozoa in im- 

 mature eggs of echinoderms and amphibians; 

 the inhibition of the division of the accessory 

 sperm nuclei in the normally polyspermic 

 eggs of salamanders and insects (cf. Fank- 

 hauser, '48) ; and the disturbances of the 

 nuclear phenomena of fertilization and of 

 mitosis in various hybrid combinations among 

 species of echinoderms and amphibians (Fig. 

 36). 



Even in the most drastic experiment, in 

 which all nuclear material is removed from 

 an egg and the developmental potentialities 

 of the egg cytoplasm are tested in isolation, 

 we cannot eliminate the egg cytoplasm's 

 history. This includes the long period of 

 development of the oocyte in the ovary dur- 

 ing which there was intensive activity of 

 the nucleus taking part in the elaboration 

 of cytoplasmic materials. Although the in- 

 terest has recently been focused anew on the 

 ovarian egg (cf. Brachet, '44, '47), much 

 descriptive and even more experimental 

 work remains to be done. 



QUANTITATIVE CHANGES IN CYTOPLASM 

 OR NUCLEUS 



CYTOPLASM 



Demonstration of Early Localization of Cy- 

 toplasmic Factors of Development. Eggs of 

 many species may be divided into two or 

 more parts by cutting, shaking, or centri- 

 fuging, before or after fertilization. The 

 part that contains the egg nucleus as well as 

 a spermatozoon usually completes fertiliza- 

 tion in normal fashion and thus begins its de- 

 velopment with the double complement of 



126 



