338 FIELDS AND GRADIENTS IN NORMAL ONTOGENY 



ment, skeletogenous mesenchyme cells and endoderm are produced 

 from the equator of the blastula, and two guts are formed, one in 

 relation to each pole. 



It is of further interest to note that the effects of lithium are seen 

 on the ventral side sooner than on other meridians, thus indicating 



Fig. 157 



A, Megacephalic, and B, microcephalic, larvae produced by exposing frogs' eggs 

 to 10 hours' adjuvant and antagonistic temperature-gradients respectively. 

 Above, external views ; below, sections of head in region of maximum brain 

 depth. Note difference in size of brain. C, Extremely microcephalic tadpole 

 produced by exposure to an antagonistic temperature-gradient for 32 hours from 

 fertilisation, then kept in water for 7 days. (Redrawn, A and B after Huxley, 

 Arch. Entwniech. cxii, 1927; C, after Tazelaar, Huxley and de Beer, Anat. Rec. 

 XLVii, 1930.) 



the existence and polarity of the dorso- ventral axis (see Chap, iv, 

 p. 68). 



All the evidence therefore goes to show that the main gradient- 

 systems are concerned with a number of separate physiological 

 processes which may be variously affected by different agencies. 

 This is an important extension of Child's views. 



