152 



The Nucleus and Cytoplasm in Development 



proximal half of the X-chromosome is lack- 

 ing there is, again, disturbed distribution of 

 cleavage nuclei, but an incomplete blasto- 

 derm is formed. When somewhat less than 

 the distal half of the X-chromosome is ab- 

 sent, a typical blastoderm develops but gas- 

 trulation and separation of the germ layers 

 fail. 



Studies of still finer subdivisions of the 

 nuclear material and of the effect of their 

 absence on development have shown that 

 even very short chromosomal deficiencies, 

 when homozygous, are lethal to the develop- 

 ing egg. When the "white" locus in the X- 

 chromosome is lacking, morphogenesis pro- 

 ceeds well into formation and differentiation 

 of the germ layers. Organs of ectodermal 

 origin appear to be more or less normal but 

 those of entodermal and mesodermal nature 

 show malformations. The sac-like midgut 

 fails to differentiate into a convoluted tube, 

 while aorta, fat body and musculature 

 either degenerate after an initial appearance, 

 or fail to be formed completely. When the 

 "facet" locus, a close neighbor of "white," 

 is absent other abnormalities of development 

 are observed (Poulson, '45). The nervous 

 system hypertrophies excessively and, while 

 showing considerable differentiation, posses- 

 ses deranged patterns of ganglia and fiber 

 tracts. The development of the mesodermal 

 and entodermal derivatives shows specific de- 

 ficiencies. 



These studies prove the necessity of the 

 presence even of minute sections of the genie 

 material if development is to proceed suc- 

 cessfully. There are a few exceptions known 

 where complete absence of small chromosome 

 sections is compatible with full development 

 although greatly weakened constitution. On 

 the other hand, for many deficiencies the 

 evidence goes beyond the recognition of the 

 necessity of the presence of the specific chro- 

 mosomal section for development as a whole. 

 Demerec ('34, '36) has investigated ho- 

 mozygous deficiencies whose lethal nature in 

 the development of the egg was known, in 

 regard to their effect on single hypodermal 

 cells in normally developing individuals of 

 Drosophila (unless specified otherwise, "Dro- 

 sophila" refers to the species melanogaster) . 

 The experiments consisted in raising flies 

 heterozygous for the deficiencies and watch- 

 ing for the appearance of cells which, in con- 

 sequence of occasional abnormal mitotic dis- 

 tribution of chromosome parts, had become 

 homozygous for the deficiencies. Many of the 

 homozygous deficiencies proved to be "cell- 



lethal," that is, no cells survived after having 

 received the completely deficient constitu- 

 tion. These cell-lethal deficiencies are thus 

 not only incompatible with development as 

 a whole but even with the carrying out of 

 minor processes of cellular differentiation or 

 of the relatively stationary processes of cellu- 

 lar metabolism. 



In other cases, homozygous deficiencies, 

 while unable to support embryonic develop- 

 ment, are compatible with survival, division, 

 and differentiation of somatic cells. However, 

 it is not known whether the chromosomal 

 section concerned is actually not required 

 for cellular metabolism, or whether the 

 breakdown of the cells is prevented by dif- 

 fusion of necessary substances into the de- 

 ficient cells from the neighboring nondefi- 

 cient tissues of the developing individual. 



The smallest genetically separable sec- 

 tions of chromosomes are called genes. The 

 experiments reported in the preceding para- 

 graphs establish the fact that genes are 

 necessary for development. These experi- 

 ments made use of the alternative: "pres- 

 ence" versus "absence" of genes. In addition, 

 the geneticist can provide the student of 

 development with other genetic variables as 

 tools for a study of the role of genes in 

 development. These tools consist of variations 

 in the quantity of genes, and in their quality. 



QUANTITATIVE VARIABILITY OF GENIC 

 CONTENT 



It is possible to obtain quantitative series 

 in which the whole genie content of devel- 

 oping eggs is multiplied from one member 

 of the series to the next: haploid, diploid, 

 triploid, etc. The effects of such differences 

 in plants have been studied extensively 

 (Wettstein, '40), and some important results 

 have also been obtained in animals (see the 

 preceding chapter by Fankhauser in this 

 volume). Usually no strikingly different ef- 

 fects on the phenotype are produced by the 

 simultaneous multiplication of all genes. The 

 developmentally "balanced" system of genie 

 action within a diploid organism retains its 

 proportionate effects in the various poly- 

 ploids. There are, however, limits to the 

 maintenance of balance. In the various mem- 

 bers of the series, the relations between nu- 

 clear and cytoplasmic volume and surface 

 are changed. These changes result in differ- 

 ences in the total numbers of cells which 

 make up specific organs, and in differ- 

 ences in both absolute and relative growth. 



