Section IV 



CHAPTER 2 



Gene Action 



CURT STERN 



THE NEED OF GENIC MATERIAL FOR 

 DEVELOPMENT 



The necessity of a nucleus for the function- 

 ing and survival of cells is, of course, a long 

 established fact. For an egg cell, this implies 

 that development cannot occur withovit pres- 

 ence of a nucleus. The few observations of 

 developmental phenomena in enucleated sys- 

 tems do not contradict this statement. The 

 partial cleavage of Amblystoma eggs whose 

 maternal nvicleus had been removed experi- 

 mentally and whose paternal nucleus under- 

 went degeneration (Jollos and Peterfi, '23), 

 or the cleavage of experimentally activated, 

 enucleated egg fragments of sea urchins 

 (Harvey, '40) can be interpreted as a residual 

 activity initiated while the nucleated egg 

 was formed in the ovary. 



This interpretation is supported by the 

 analysis of development of fruiting bodies in 

 the unicellular marine green alga Aceta- 

 bularia mediterranca and related species (or- 

 der Siphonocladiales) (Hammerling, '34, 

 '46). In the juvenile state these forms consist 

 of a rhizoid containing a single, large nu- 

 cleus and a stalk up to 7 cm. in length. On 

 reaching maturity the anterior end of the 

 stalk develops a complicated umbrella-like 

 fruiting body. Early removal of the nucleus, 

 by excision of the rhizoid, inhibits completely 

 the development of the umbrella. Late re- 

 moval of the nucleus is compatible, in many 

 cases, with qualitatively complete though 

 quantitatively diminished development of 

 the complex morphological featvires of the 

 umbrella, but more frequently no or only 

 incomplete morphogenesis results. Reintro- 

 duction of a nucleus into the enucleated sys- 

 tem completely restores the developmental 

 capacity. Clearly, some nucleus-dependent 

 substance or substances are instrumental in 

 these developmental processes. The cytoplasm 

 of the giant cell can store these substances 

 and use them for partial morphogenesis but 



it is idtimately dependent on the nucleus 

 for their production. 



The first step in the analysis of the sig- 

 nificance of nuclear constituents for de- 

 velopment was made by Boveri ('02, '07). 

 The causes of abnormal development of 

 dispermic sea-urchin eggs were traced to the 

 presence of nuclei in which certain chromo- 

 somes were lacking, and different types of 

 abnormal development were linked with ab- 

 sence of different chromosomes. The nature 

 of the experimental material — the 18 chro- 

 mosomes of the haploid set of the sea urchins 

 were not distinguished individually — did not 

 permit that specific developmental assign- 

 ments could be made to specific chromo- 

 somes. 



In Drosophila mclanogaster the necessity 

 of having each of the four kinds of chromo- 

 somes represented in the fertilized egg in 

 order to produce successful development has 

 been proven by the use of genetic techniques 

 which involve the "marking" of each kind 

 of chromosome with specific genes (Li, '27, 

 and earlier indirect evidence: Morgan, 

 Bridges, and Sturtevant, '25, p. 135). An 

 embryological study of the effect of the 

 absence of the X-chromosome showed early 

 abnormalities in the distribution of the cleav- 

 age nuclei (Poulson, '40, '45). No blastoderm 

 is formed and neither morphogenetic changes 

 nor differentiation occurs. Cleavage and cell 

 divisions proceed for several hours before 

 degeneration sets in. 



We have no similar observations on Dro- 

 sophila eggs lacking autosomes, and we can 

 only assume but not prove that the break- 

 down of development would be different in 

 type from that caused by lack of the X- 

 chromosome if another chromosome were 

 absent. That this assumption is probably cor- 

 rect can be judged by studies of eggs in 

 which either one of two complementary sec- 

 tions of the X-chromosome are absent (Poul- 

 son, '40). When somewhat more than the 



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