" DOUBLE MONSTERS IN FISHES 69 



in which the germ ring plays no part in the formation of 

 the embryonic axis. This means that the two bilateral 

 primordia are formerly more or less separate and that 

 at the time of gastrulation, or the formation of the axis 

 of symmetry, there is a highly energetic coming together 

 of the cells of the two sides so as to converge in a median 

 dorsal line. The energy of concrescence is greatest at 

 the anterior end, as may be determined by susceptibility 

 experiments, and progressively less great farther down 

 the axis. If at the time when the process of concrescence 

 is going on most actively the rate of metaboUsm is 

 markedly lowered, or even if the actual interruption of 

 development has been earlier and its effect still persists 

 at the time of gastrulation, this process may be more or 

 less inhibited. If the inhibition is slight it may affect 

 only the most susceptible structures such as the forebrain 

 and the eyes; if a little more severe, certain dorsal or 

 median anterior structures may be prevented from 

 concrescing; if considerably more severe, the effect may 

 be felt far down the axis and all structures except certain 

 median ventral ones may be divided. This view is 

 entirely consistent with the facts. It rationalizes the 

 symmetrical arrangements of the divided primordia, 

 for if the notochords or neural tubes are the products of 

 the bilateral fission of a single primordium, what more 

 natural than that they should be equal and symmetri- 

 cal ? It rationalizes the observations that situs inversus 

 viscerum is of very common occurrence, for if the two 

 components of conjoined twins are derived from the 

 bilateral primordia of a single individual we might 

 expect to find such evidences of mirror-image symmetry.' 

 This view is essentially one of the physiological isolation 



