SECTIONAL TRANSACTIONS.— D. 365 



divergent on their importance to the poultry industry. What little work 

 has been done, however, suggests that light or moderate infestations cause 

 no discernible injury, although heavy ones may. Treatment of whole 

 flocks with drugs is, therefore, unwarranted unless an unusually heavy 

 infestation is known to be present. 



Until more satisfactory experimental evidence is forthcoming on the 

 pathogenicity of these parasites and the significance of multiple parasitism, 

 the economic importance of these widely distributed parasitic worms will 

 remain a matter for conjecture. 



Other speakers, including Mr. A. J. Macdonald, Mr. J. Wilson, 

 and Dr. A. L. Romanoff. 



Tuesday, September 15. 



Discussion on Morphogenesis (10.0). 



Mr. C. H. Waddington. — Organisers and their limitations. 



An embryonic organ has, at any given time, a definite shape, and as time 

 progresses the shape alters in a determinate way into a new shape of greater 

 complexity. The influences which bring about these changes of shape must 

 themselves be arranged in an orderly way in space, and the whole set of 

 forces which control the shape of an organ may be spoken of as the field- 

 forces of that organ . The paper will discuss the general concept of embryonic 

 fields, with particular reference to the field-forces of the neural plate of the 

 Amphibia and the way in which they are dependent on the underlying 

 mesoderm. 



Dr. P. D. F. Murray. — Morphogenetic fields and self-differentiating 

 mosaics (10.30). 



Morphogenetic fields are characterised by their integration as units, by 

 their ability to incorporate indifferent material within themselves, and by 

 their ability to regenerate parts of themselves. In the development of the 

 chick embryo jaw the determination, or evocation, of tissues occurs in three 

 separate regions, presumably by three acts of evocation. The loci of de- 

 termination are not fields but areas, though they may constitute a field 

 together with the organiser responsible for their evocation. In later 

 histogenesis, field phenomena are less easily recognisable. In its morpho- 

 genesis, the chick embryo limb-bud is a mosaic, without obvious relations of 

 dominance and subordination between its parts, and almost lacking in the 

 other field characters. Nevertheless, it is probable that some regeneration 

 and regulation may occur, and phenomena characteristic of fields are 

 recognisable in joint formation. Evocation is the event which occurs in 

 the determination of cells to form kinds of tissue, the first stage in histogenesis. 

 Individuation is the integration of determined tissue so that the self- 

 differentiating mosaic is brought into existence, and the parts of the organism 

 concerned in both processes may constitute fields because of the relations 

 of dominance and subordination which exist between them. 



Prof. E. A. Spaul. — Endocrines and morphogenesis (11.0). 



