II ORGANOGENESIS 731 



technique has been used many times since (Boell, 1948; Clark, 1953a; Clark, 

 Florio and Hurowitz, 1955; Clark and Sisken, 1956). Levy (1952) makes a further 

 modification, based on the concept that a logarithmic, rather than linear, 

 expression of time is more appropriate for physiological events. He points out 

 that such log-log plots of various embryonic dimensions (weight, nitrogen, 

 dipeptidase, aminopeptidase, cytochrome oxidase and diphosphopyridine nucleo- 

 tides) have nearly identical time relations to each other and to the totality of 

 development. A log-log plot results in a straight line with a change in slope 

 corresponding to change in rate of accumulation of the units under consideration. 

 The variables mentioned above have changes in rate between 4.3 and 4.5 days 

 in the chick. 



b. Cessation of growth. Conceptually, it might be easier to devise mechanisms for 

 alteration in growth rate of a particular component of the embryo than to provide 

 a device for stopping growth. For it is a common experience, at least in warm- 

 blooded animals that there is a constant species size of the organism and that, 

 within the organism, the relative sizes of the several organs is constant {e.g., 

 Latimer, 1925, 1947, 1951, 1952 and many other similar studies). Some experi- 

 mental attempts have been made to provide explanation of a mechanism, appa- 

 rently under genetic control, for stopping the growth process "at the right time"'. 

 Weiss (1947) proposes such a mechanism. Its basic features are: (i) Existence 

 of intracellular "template" molecules, (2) proportionality of growth to the number 

 of template molecules present, (j) confinement of template molecules to the cell, 

 and {4) production by template molecules of antitemplates, which can escape 

 the cellular barrier, (5) cessation of outward migration of antitemplates when 

 equilibrium is reached across the cell membrane, and (6) fixation of intracellular 

 antitemplates by templates and prevention, thereby, of further template pro- 

 duction (growth). He supports this thesis (1952) by an in vitro experiment in 

 which embryonic heart and kidney were cultured in extracts of whole embryos, 

 or of whole embryos less the homologous organ. The results were as follows: 

 (j) heart — of 333 in whole extract 2 pulsated after the fourth day; of 349 in 

 deficient extract, 129 pulsated; (2) of 1007 kidney explants 74 formed tubules in 

 the growth zone in whole embryo extract, whereas of 1006 explants in deficient 

 extract, 176 formed tubules; (j) on the other hand, injection of embryonic kidney 

 "brei" into younger embryos caused a 50%-increase in mitotic count in the host 

 kidney, and removal of one embryonic kidney resulted in hyperplasia of the 

 remaining kidney with more than 100% increase in mitotic activity after 48 h. 

 Rose (1952) makes a proposal along similar lines in which he conceives of 

 growth limitation as a mass action phenomenon, whereby synthesis of a new 

 product would be limited by the quantity of that product already formed. In 

 support of his thesis he oflfers some data on the eflfect on amphibian differentiation 

 when the embryos are reared in a medium to which specific extracts of adult 

 organs have been added. Under such conditions he reports inhibition of brain 

 growth by brain extract, heart and circulatory diflferentiation by heart extract. 

 Shaver (1954) also submits data showing inhibition of nervous tissue in the frog 

 by tissue fractions of the adult organ. 



Literature fi. 744 



