3l8 FIELDS AND GRADIENTS IN NORMAL ONTOGENY 



point of a secondary gradient, established after fertilisation, and 

 the precise morphogenetic effects are due to the interaction of this 

 with the original animal- vegetative field established in the oocyte. 

 The relation between the gradient-field set up by a dominant 

 region and an amount of tissue representing a reduced range, is 

 well shown in the experiments on newt embryos (described on 

 p. 239) in which the early gastrula is constricted into dorsal and 

 ventral halves. The dorso- ventral gradient is then of half the 

 normal length, and the dorsal half-gastrulae possess neural folds of 

 proportionately reduced size. Another example is provided by the 

 experiments on sea-urchin larvae to be described below (p. 323), in 

 which four micromeres are added to a single ring of mesomeres 

 (disc an i), and a properly proportioned pluteus larva is formed. 

 The main gradient is here represented by one quarter of its original 

 length, and in this case the amount of the dominant region has had 

 to be reduced in order to produce a harmonic result. 



§ 3. The interaction of primary and secondary gradients 



In early amphibian development, for instance, there appears 

 clearly to be two gradients of qualitatively diflFerent nature. One 

 is the gradient along the primary egg-axis from animal to vegetative 

 pole ; the other, a gradient whose high point or dominant region is 

 the organiser. The first appears to be established during the de- 

 velopment of the oocyte in the ovary. It must in the first instance 

 be quantitative and concerned only with some general activity of 

 the cytoplasm : but by the time that the egg is ripe, it has in addition 

 produced a structural effect, in the shape of the graded increase in 

 the proportion of yolk found when passing down the egg-axis to- 

 wards the vegetative pole. The existence of this gradient has been 

 shown by susceptibility experiments. (See p. 332, and figs. 154, 



i55> 156.) 



Per contra, although the other gradient, which is normally 

 established as a result of fertilisation, has a sharply qualitative 

 aspect in that the dorsal lip region alone is capable of exerting 

 organiser capacities, yet it is also quantitative in other aspects. For 

 instance, it is found that, as determined by cell-size in the late 

 blastula and early gastrula, the rate of cleavage in the future dorsal 

 side of the animal hemisphere is greater than in the ventral side. 



