FIELDS AND GRADIENTS IN NORMAL ONTOGENY 327 



Partial or regional fields can also give rise to more than one 

 structure. The amphibian organiser region itself can be divided 

 and engrafted to produce several embryos (p. 151). The limb-field 

 can be made to produce a number of limbs. This can be done not 

 only by grafting portions of it into new situations (p. 223), but 

 simply by making deep cuts in the early Kmb-buds:^ the result 

 is a number of limbs growing out from the limb-area. Other 

 regional fields also show this multiple potentiality, e.g. heart, 

 balancer, etc. One of the most striking examples is provided by the 

 anterior end of a Planarian, which, by making deep cuts, can be led 

 to give rise to as many as ten heads^ (fig. 151). 



One point which may here be mentioned is the existence in all 

 large-yolked vertebrate embryos and in all mammals of consider- 

 able areas of tissue produced by the fertilised egg but not organised 

 into the body of the embryo. Examples of such tissues are the 

 extra-embryonic blastoderm of selachians, reptiles and birds, and 

 the trophoblast of mammals. These do not appear to be organised 

 in relation to the organising centre of the embryo, and in some 

 cases (chorion or trophoblast of amniotes) are cast away at hatching 

 or birth, and thus never become incorporated in the field-gradient 

 system of the organism. In other cases (yolk-sac) they do ulti- 

 mately become incorporated by resorption within the body, and 

 are then organised to produce a portion of the gut. 



Such extra-embryonic structures may perhaps be looked on as 

 composed of tissue which has grown so rapidly as to escape the . 

 organising action of the organiser, and thus to remain beyond the 

 boundaries of the embryo. It is of interest that exposure of fowl 

 eggs to low temperature will produce a large proportion of '*ani- 

 dian" blastoderms, in which no embryo is formed, but the blasto- 

 derm shows considerable powers of growth.^ 



With regard to points (viii) and (x) of our previous chapter, these 

 only apply to cases of regeneration. They are thus not relevant to 

 normal ontogeny. 



These points lead on to a consideration of the problem of 



twinning. The term twinning in the broad sense is applied to any 



process by which more than one individual is produced during 



early ontogeny from a single zygote. We may, however, profitably 



1 Tornier, 1906. ^ Lus, 1924. ^ Needham, 1933. 



