FIELDS AND GRADIENTS 309 



results.^ It appears that at room temperature the formation of a 

 head is determined in about 6 hours from the time of operation. 



§5 



There is another fact concerning the gradient-systems of adult 

 lower invertebrates which requires consideration, for it throws light 

 on certain processes of embryology. This is the double gradient 

 analysed by Child and his school in Annelid worms. In these 

 animals, as is well known, new segments are added from a growing 

 zone in the penultimate segment of the body. Experiments with 

 dilute toxic solutions show that there is a region of high susceptibility 

 at both ends of the worm, with a minimum at an intermediate point. 

 Child and his school have always attempted to reduce all gradient- 

 phenomena to variations in a single variable, which they have tried 

 to identify with oxidative metabolism, but which, theoretically, 

 might be any general activity of protoplasm. This conception, how- 

 ever, seems definitely to break down in face of the facts in Annelids. 

 Here, two distinct processes appear to be at work. One is the forma- 

 tion of new segments at the hind end associated with the presence 

 of undifferentiated, physiologically young tissue ; the other is the 

 controlling and morphogenetic activity of the front end, associated 

 with old tissue and a high grade of differentiation. It is worth re- 

 calling that the conditions of formation and the morphogenetic 

 effects of the dominant region in Annelids are similar to what is 

 found in Planarians (see p. 279). If regeneration occurs at all at an 

 anterior cut surface, the normal result is a new dominant region, 

 which never consists of more than a small number of segments, 

 constant for each species ; and this, once produced, causes morpho- 

 genetic changes in the old tissues, such as the production of a new 

 genital region at the correct distance behind the head, or the trans- 

 formation of a certain length of intestine into crop and oesophagus, 

 or the conversion of abdominal segments into thoracic segments." 

 Though both head and tail in Annelids are regions of high sus- 

 ceptibility, the processes at work in the two are entirely distinct. 

 There are therefore two qualitatively different gradients in the 

 organism, and there is every right to believe that the effects of the 



^ Child, 1914. See also Abeloos, 1932. 

 ^ Harper, 1904; Berrill, 1931. 



