SEPARATION OF GROWTH AND DIFFERENTIATION 



161 



WEISS, P. , 1947 - "The problem or specificity in growth and development. " Yale J. Biol. & Med. 19:235. 



YOUNGSTROM, K. A. , 1938 - "On the relationship between choline esterase and the development of behavior in Amphibia. " 



Jour. Neurophysiology. 1:357. 

 YUNG, E. , 1885 - "De I'influence des variations due milieu physicopchemique sur le developpement des animaiix. " Arch, des 



Sci. Phys. et. Nat. 14:502. 



Fig. 1. - Embryo embedded in the late gastrula stage exhibits, 8 days later, the original 

 egg shape but has developed stunted external organs. 



Fig. 2. - The ventral portion of an embedded neurula has grown through a hole in the 



agar investment, while the upper portion has not increased in volume but con- 

 tinued differentiating. 



Fig. 3. - The embryo has for 12 days retained the general shape existing at the time of 

 embedding. Left nasal placode and right gill region form abnormal extensions 

 into holes cut into the agar matrix. 



Figs. 4, 5, 6. - Differential inhibition of development in blastulae of Rana fusca that have 

 been kept for 3 days closely packed in between other eggs. The frontal portion 

 which alone had free access to the immersion fluid is least inhibited showing a 

 medullary head fold and a sucker (Figs. 4-5). In the absence of exogastrula- 

 cion such eggs develop into microcephalic larvae (Fig. 6). 



(From Holtfreter, 1945: Anat. Rec. 93:S9) 



"The determitiation or chemo-differentiation of any given part (i. e. , 

 the decision as to what any cell or group of cells sliall develop into) takes 

 place invisibly some time prior to the process itself. " 



H. Spemann 



