TECHNIQUES OF ANATOMY — ROOFE AND LESHER 331 



is not an isolated case, for it has been found that the entire nervous 

 system of vertebrates is called forth by an inductive act. The recent 

 work of J. Holtfreter (Rochester, Minn., a student of Spemann) and 

 J. Needham (Cambridge, England) indicates that the inductive proc- 

 ess is mediated by a chemical substance. There is considerable dis- 

 agreement as to its exact composition in which speculation concerning 

 its nature ranges from certain proteins, fatty acids, and nucleic acids 

 to Needham's steroids. 



The discovery of the organizer by Spemann and Hilde Mangold 

 (1924) was of extreme importance to the experimental embryologist. 

 It was found that when the upper lip of the blastopore, during the 

 process of involution, was cut out and transplanted to the flank of 

 another gastrula it proceeded with its invaginating movements. 

 When this transplanted material reached the inside of the gastrula it 

 underwent differentiation into a complete or partial embryo. This in- 

 ductive force in itself is not unusual, but the most remarkable feature 

 was that all the structures so originated were combined to form a 

 whole, well-integrated, secondary embryo on the host. This organizer 

 action is essentially and fundamentally a combination of self-differ- 

 entiation and of complex induction. 



These studies on the nature of the organizer and the inductors led 

 quite naturally to the origination of a new type of experimental em- 

 bryology, one which Needham calls "chemical embryology," but be- 

 cause of the broadness of its scope it could more appropriately be called 

 "physiological embryology." At first the investigations were purely 

 chemical and of an analytical nature, e. g., the chemical analysis of 

 whole eggs. Recently attention has been centered on the enzyme sys- 

 tems as they become active during the process of development. It is 

 here that the concepts of the chemist, the cytologist, the geneticist, and 

 the embryologist merge. Physiological embryology is the basic or 

 unifying branch of the whole field of embryology. 



CYTOLOGY 



Cytology deals with the structure of cells. To the biologist the 

 cell is what an atom is to the chemist and physicist, i. e., it is a funda- 

 mental morphological and physiological unit in the structure of living 

 organisms. The development of this biological discipline has for the 

 most part coincided with the development of the compound micro- 

 scope. The earlier nineteenth-century cytologists were, however, con- 

 cerned almost entirely with descriptive or morphological cytology. 

 The role that cytology was destined to play in cell physiology did not 

 begin to unfold until the establishment by the Morgan School at Co- 

 lumbia University of the chromosome theory of inheritance. This 



