H. E. LEHMAN 203 



fragments of echinoderms and amphibians have the capacity to 

 cleave and form "blastulae," but none has given any evidence of 

 gastrulating (see reviews by Fankhauser, 1955; J. A. Moore, 

 1955 ) . The rate and pattern of cleavage until the onset of gastrula- 

 tion in hybrids, and particularly in hybrid andromerogons, pro- 

 ceeds strictly according to the genetic character of the egg 

 cytoplasm and gives no indication of sperm nucleus influence 

 until gastrula and postgastrula stages (see reviews by Needham, 

 1950; Fankhauser, 1955; J. A. Moore, 1955). Physiological and 

 biochemical studies indicate that, although there is a continuous 

 rise in the metabolic rate during cleavage, the most conspicuous 

 accelerations in respiration and specific syntheses follow the onset 

 of gastrulation (see reviews by Barth and Jaeger, 1947; Brachet, 

 1947, 1952; Gregg, 1948; Ebert, 1954; Fankhauser, 1954). These 

 observations suggest that, if nuclei become differentiated during 

 development, such changes should be sought, not in early devel- 

 opment when the nuclear influence is slight but, rather, in gas- 

 trula and older tissues when nuclear function can be clearly 

 demonstrated. 



In examining the possibility of nuclear differentiation, the first 

 question which must be answered relates to whether or not there 

 is sufficient evidence to indicate that cytoplasmic differentiation 

 is accompanied by tissue-specific nuclear change. Until this is 

 settled, it is premature to speculate on secondary questions relat- 

 ing to mechanics of nuclear differentiation, or labile versus ir- 

 reversible determination, or functions of differentiated nuclei 

 in histogenesis and tissue syntheses. Present information does 

 not allow one to generalize in answering the primary question; 

 however, certain evidence unequivocally shows that at least some 

 differentiated somatic cells possess nuclei which vary in a tissue- 

 specific manner from other nuclei in the organism. 



Evidence for Nuclear Differentiation in Somatic Tissues 



The nuclei of differentiated tissues possess distinctive varia- 

 tions in size, shape, density, and composition which are histologi- 

 cally diagnostic for specific tissues. Many of these differences 

 show a constant relation to functional activity of the cells con- 



