6 THE PHYSIOLOGY OF TWINNING 



tardation does not seriously affect the blastoderm until 

 the process of gastrulation begins, a process of extreme 

 delicacy, as has often been pointed out. Once fairly 

 started the cleavage process seems to be easily accom- 

 plished even under great difficulties. This was clearly 

 brought out by the writer (Newman, 191 5) in his work 

 with teleost hybrids. 



Almost any cross, even those between the most 

 distantly related species, will go through cleavage nor- 

 mally or nearly so, but, because of the relative slow- 

 ness of the cleavage process, the important crisis of 

 gastrulation is the usual halting-point for such hybrid 

 embryos: their developmental momentum is insufficient 

 for them to accomplish gastrulation. Now gastrulation 

 results in the establishment of the embryonic axes, 

 including the axis of polarity and the axis of symmetry. 

 The result is that if gastrulation is only temporarily 

 halted the process of formation of the axis of symmetry, 

 or the bilateral axis, may be so interfered with that the 

 primordia of the two equivalent halves of the sym- 

 metrical embryonic axis may become more or less 

 completely isolated physiologically, and each half may 

 produce a more or less perfect whole embryo in the region 

 where the isolation has occurred. 



INTERPRETATION OF DOUBLE MONSTERS (COSMOBIA) 



My interpretation, therefore, of double monsters, 

 at least of those of a symmetrical sort which have certain 

 of their central or axial structures united, is that they 

 are products of a partial twinning process involving a 

 separation of equivalent right and left parts of an 

 originally single embryonic axis. This is opposed to the 



