330 FIELDS AND GRADIENTS IN NORMAL ONTOGENY 



(fig. 153), in Patiria after fertilisation by sperm of another species, 

 or as a result of overcrowding. 



In all cases in which twinning has been experimentally produced, 

 it is clear that the critical stage at which dichotomy occurs is that 

 of early gastrulation. In the reversed frog's egg the invaginated gut 

 becomes mechanically split into two in a manner described above 

 (p. 95) and since the gut-roof is the organiser, the resulting em- 

 bryo is accordingly more or less completely doubled. Similar cases 

 are operative in the production of anterior doubling as a result of a 

 ligature constricting the tgg in the plane of bilateral symmetry 

 (p. 156, and figs. 32, 152, 169, 170). 



In other cases, the twinning is due not to a physical but to a 

 physiological dichotomy, and the region aff"ected appears always to 

 be the apical point of a gradient. This point is known to be dif- 

 ferentially susceptible to depressants (p. 332). All agencies which 

 make for abolition of polarity, by reducing the rate of activity of the 

 apical point and flattening the gradient, also tend to encourage the 

 production of twinning. 



This is particularly well seen in Patina where as a result of a 

 lowering of the general rate of activity consequent upon abnormal 

 fertilisation or overcrowding, invagination of an enteron takes place 

 not from one, but from two or three points.^ The same phenome- 

 non occurs in teleosts (Fundulus and trout), where as a result of the 

 depressant effects of cold, or lack of oxygen, the originally single 

 axis of polarity is replaced by two.^ 



In the armadillo, there is, relatively to other mammals, a delay in 

 the formation of a placenta, and consequently in the establishment 

 of a source of supply of oxygen and nutriment for the embryo, and 

 this occurs at a stage corresponding to the early gastrula, just before 

 the appearance of the primitive streak. In those occasional cases in 

 which two embryos are formed on a single blastoderm in a bird's 

 egg, it is probable that the cold experienced by the egg after laying 

 and before incubation is responsible for an arrest of development 

 at a stage which corresponds to the early gastrula, shortly before the 

 appearance of the primitive streak. 



The twinned worms occasionally to be found in the cocoons of 

 Oligochaetes are presumably to be accounted for by a delay in 

 ^ Newman, 1923. ^ Stockard, 1921. 



