DEVELOPMENT AND DIFFERENTIATION 



of generations which seems to present the organism with more 

 difficiik problems than its suppression. In each pollen grain or 

 embryo-sac or fertilized egg a nucleus of a new type produced by 

 mciosis or fertilization is suddenly implanted in a cytoplasm accus- 

 tomed to, associated with, or generated by, a previous and different 

 nucleus. This situation requires, as we have already seen, that the 

 cytoplasm has to take the lead in deciding the course of development, 

 sporophytic or gametophytic. It also has the consequence that these 

 cells are the most sensitive of aU to upset in the relationship of 

 nucleus and cytoplasm. In the case of embryo-sac and egg the large 

 bulk of the cytoplasm, or the continued nursing by the mother 

 tissue, relieves the crisis. But the pollen grain has to fend for itself. 

 It is the pollen grain, therefore, which is the first to suffer for malad- 

 justment, Male-sterility is the prevailing failure of plant hybrids and 

 the prevailing outcome of cytoplasmic mutation. Of this principle 

 we have seen some instances and we shall see more. 



How the Nucleus Acts: Lag 



The determination of gametophytic or sporophytic development 

 is immediately cytoplasmic, but the observations of Andersson- 

 Kotto on Scolopendrium vulgare show that it is ultimately genie. A 

 single dose of the gene "peculiar" turns some of the spores into 

 sperm. A double dose has the still more drastic effect of short- 

 circuiting the reproductive cycle by causing the sporophyte to 

 develop gametophytic sprouts. Similar transformations are, of course, 

 a commonplace of genetics. Any number of genes are known which 

 convert stamens into petals or carpels, petals into stamens or sepals 

 and so on. In Drosophila one gene will restore the four wings of its 

 ancestor, while another will turn the feelers, and another again the 

 mouth parts, into legs. Thus, in the very process of proving that the 

 cytoplasm is immediately responsible for differentiation, we are 

 compelled to show that nuclear genes exercise the ultimate control. 

 The cytoplasm is their agent. 



The way in which the nucleus acts through the cytoplasm has 

 been well displayed by Hiimmerling's experiment with the single- 

 celled Acctahularia to which we have already had cause to refer. An 

 individual deprived of its hat will grow a new one. It wiU still do 



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