38 THE PINEAL ORGAN 



may be produced experimentally by splitting the anterior end of an early 

 larva and preventing the two halves from uniting. Each head will show 

 bilateral symmetry and develop the normal pair of eyes and sensory 

 organs. 



Thus two organizing centres may be obtained after the stage when a 

 single normal organizer has already been established. Further, it is 

 well known that twin or even multiple gastrulae may occur in normal ova : 

 as for example, in the Texas armadillo, which has four primitive streaks, 

 and a South America species of armadillo, which has eight, and normally 



Fig. 30. — Larva of Fundulus Heteroclitus. 



A — Normal larva with anteriorly placed mouth. 



B — Incomplete cyclopean larva ; the two eyes are joined, and occupy the position 



usually taken by the mouth. 

 C — Complete cyclopean larva, with antero-median eye. Dorsal aspect. 



M : mouth. 



(After Stockard, from Dendy's Outlines of Evolutionary Biology.) 



eight offspring at a single birth. These are believed to arise by the sub- 

 division of a single blastoderm, and to be comparable with the artificial 

 dichotomy of a growing bud and possibly also the induction of twinning in 

 lower animals by merely separating the primary blastomeres ; or the 

 production of separate embryonic axes in the eggs of fishes by lowering the 

 temperature or reducing the oxygen supply during the early stages of 

 segmentation. 



There is, in fact, abundant proof of the uniovular origin of true twins. 

 Further, the occurrence of multiple births arising from a single ovum, 

 as in the case of the two species of armadillo, is well known to be an 



