yiG NITROGEN METABOLISM AND GROWTH 9 



and even later, embryologists were largely preoccupied with the infinite variety 

 of questions related to the "organizer" and inductive phenomena. The metho- 

 dology of these pursuits provided primarily information with regard to higher 

 levels of organization: interrelationships between tissues, establishment of "fields" 

 (Weiss, 1939) and metabolic behavior of specific areas of the developing embryo 

 (Child, 1941; Brachet, 1950). 



Although important work on behavior of tissues and larger cellular aggregates 

 does, and must, continue, several factors have combined in focussing attention 

 of embryologists on intracellular details during development. Among these must 

 be included the search for the identity of "the organizer" of amphibia (see 

 Brachet, 1950, for review) which inevitably led to the examination of intra- 

 cellular behavior both in the inducing cells and in the responding cells; contem- 

 poraneously interest in physiological genetics grew rapidly (Goldschmidt, 1938) 

 leading to pursuit of genie mechanisms for inheritance of metabolic patterns (see 

 "Wright, 1941 ; Beadle, 1946; Wagner and Mitchell, 1955), cytoplasmic inheritance 

 (Sonneborne, 1950; Beale, 1952; Caspari, 1948, 1956), specific application of 

 genetic methods to development through study of phenocopies (Landauer, 1948, 

 1952a, 1953) and lethal mutations (Gluecksohn-Waelsch, 1954, review; Hadorn, 

 1948, 1956), hybridization (Moore, 1950) and exchange of nuclei (Briggs and 

 King, 1955). 



Technical advances, as well as conceptual orientation have contributed to 

 more critical evaluation of the biochemical and physiological interpretation of 

 development. Among these may be included metabolic studies with radioactive 

 and stable isotopes, serological techniques, microbiological assay, tissue culture, 

 histochemical and cytochemical methods, electrophoresis, immunophoresis, paper 

 chromatography and the use of metabolic inhibitors and analogs. As a result 

 of these, often used in combinations of various sorts, a basic appreciation of the 

 problems of development at the cellular level is emerging. 



The topic is complex; the investigative methods are varied; where inter- 

 relationships seem conclusive, we shall insist upon them; where discordant results 

 cannot be reconciled we shall point them out. The delicately balanced and 

 integrated systems of the developing organism will countenance few incongruities; 

 their existence in the data, therefore, writes boldly the advice of unfinished 

 business for the scientist. 



II. NITROGEN METABOLISM IN RELATION TO ASPECTS OF DEVELOPMENT 



To coordinate, in so far as possible, traditional morphological observations with 

 metabolism of nitrogen compounds, the data will be presented with reference to 

 gametogenesis and fertilization, cleavage and blastulation, gastrulation and 

 organogenesis. The fact that certain of the data have a meaning which transcends 

 artificial subdivisions of a continuous, organized process is expected; the difficulty 

 of reconciling information pertaining to the same stage in different animals may 

 simply point up the fact that the designation of stages is semantically convenient, 

 but biologically accurate in only a superficial way. 



