Energy Exchange and Enzyme Development During Embryogenesis 547 



cell functions in a manner similar to the 

 ATPases in non-contractile tissue (Barth and 

 Jaeger, '47a,b, '50a,b; Barth and Barth, '51; 

 Flexner and Flexner, '48; Potter, Schneider, 

 and Liebl, '45; Moog, '47). 



The accumulation in tissues of alkaline 

 phosphatase has been studied by Moog ('44a, 

 '50, '51, '53). Undifferentiated tissue is rela- 

 tively rich in alkaline phosphatase, but it 

 then declines during differentiation of those 

 tissues in which it has no obvious functional 

 significance. On the other hand, in tissues 

 where the enzyme presumably functions in 

 some way, alkaline phosphatase undergoes 

 tremendous increase in activity. This is well 

 illustrated in the chick duodenum. Phos- 

 phatase activity is very low until the ninth 

 day of incubation, but then it increases tre- 

 mendously, reaching a peak on the day of 

 hatching. In the mouse, there are two peri- 

 ods of phosphatase increase; the first begins 

 on the sixteenth day of gestation, and, by 

 the time of birth, an increase of a hundred- 

 fold has resulted. A second rise begins on the 

 fourteenth or fifteenth day post partum, and 

 this is of the order of twentyfold. Thus the 

 total increase in activity is some 2000-fold. 

 The first increase, Moog believes, is in 

 preparation for the diet of the mouse during 

 the nursing period; the second anticipates 

 the varied solid diet of the mouse on wean- 

 ing. A similar case of enzyme development 

 in "anticipation of function" is seen in the 

 case of tyrosinase in the grasshopper embryo. 

 The enzyme develops to its full activity dur- 

 ing the pre-diapause period, yet the first 

 indication of its function in the formation 

 of melanin pigment is not apparent until a 

 short time before hatching (Bodine and 

 Boell, '35; Bodine, Allen, and Boell, '37). 



Perhaps no better example of the intimate 

 relationship between functional differentia- 

 tion and enzyme development exists than 

 that between cholinesterase and the nervous 

 system. Youngstrom ('38) and Nachmansohn 

 ('39) had shown that cholinesterase increases 

 some time during the period when the nerv- 

 ous system becomes active, but it was left 

 for Sawyer ('43) to demonstrate the precision 

 with which neuromuscular development and 

 increase in cholinesterase activity are cor- 

 related. Sawyer showed that the concentra- 

 tion of the enzyme, low in premotile stages 

 of Amblystoma punctatum, rose significantly 

 and progressively during development of the 

 behavior seqvience described by Coghill ('29). 

 These relationships are shown in Figure 204. 



Through determinations of cholinesterase 

 in the major subdivisions of the nervous sys- 



tem, it was possible to show, as might be 

 predicted from Coghill' s study of the ontog- 

 eny of neuromuscular activity, that cholin- 

 esterase appears first in the spinal cord, and 

 then sequentially in hindbrain, midbrain, 

 and forebrain. Increase of the enzyme in the 

 spinal cord coincided exactly with the first 

 ability of the embryo to respond neurogeni- 

 cally to an external stimulus. In the midbrain, 

 cholinesterase development was most marked 

 at the time when motor activity of the cord 

 began to be dominated by the mesencephalon, 

 as shown by Detwiler's studies ('46a,b). The 

 relationship between differentiation of the 

 nervous system and cholinesterase is further 

 illustrated by the results of experiments in 

 which the extent of neural differentiation was 

 influenced by experimental means. It is well 

 known that removal of the eye from one side 

 of the amphibian embryo, before optic fibers 

 have developed, leads to marked hypoplasia 

 of the midbrain lobe on the side opposite to 

 that of eye removal. That this involves a 

 qualitative change in the neural tissue, i.e., 

 reduction in number of differentiated neu- 

 rons, has been shown by a number of workers. 

 Determination of cholinesterase in the two 

 halves of the midbrain, at various times 

 after unilateral eye extirpation, reveals that 

 cholinesterase activity (per Mg. nitrogen) de- 

 velops normally in the lobe that receives 

 optic fibers. But in the opposite side (deprived 

 of incoming fibers from the retina), cholin- 

 esterase activity is much reduced, and the 

 relative decrease is roughly proportional to 

 the reduction in number of differentiated 

 neurons. The decrease in cholinesterase ac- 

 tivity appears to be confined to the midbrain, 

 for the enzyme activities in the right and 

 left sides of the diencephalon and hindbrain 

 were found to be identical. Furthermore, the 

 effect on cholinesterase seems to be specific, 

 for the respiratory and succinoxidase activi- 

 ties of the two lobes of the midbrain were 

 also found to be the same (Boell and Shen, 

 '49, '50). 



The question may be asked whether the 

 changes in enzyme activity described above 

 are simply general effects of increased devel- 

 opment of an organ or whether they are 

 specifically and directly associated with the 

 development of function in that organ. If 

 the former possibility is correct, one might 

 suppose that many other enzymes and chem- 

 ical entities wovdd vary in the same way 

 during functional differentiation; if the lat- 

 ter is true, one would expect the ontogeny 

 of specific enzymes to be independent of 

 other more general biochemical changes. The 



