110 The Philippine Journal of Science i9i6 



developing egg, advanced the theory of polyspermy — ^that is, they 

 believed that two or more sperms might fertilize an ovum, the 

 result of which would be twins, triplets, or quadruplets, depend- 

 ing on the number of spermatozoa participating in the fertiliza- 

 tion of the ovum. This, however, has never been substantiated. 

 Moreover, since it has been shown that the chromosomes play 

 such a definite part in cell division, the theory that twins are 

 produced by polyspermy can be discarded. 



Mall has pointed out that the first observations of any value 

 on the development of twins and of double monsters were made 

 by Vejkousky(5) in 1891. His experiments were made with 

 the eggs of Allolobophora trapezoides, a common earthworm. 

 He found that when the eggs were placed in water and subjected 

 to a higher temperature segmentation began at once. At the 

 two- or four-cell stage, however, the cells would partially separ- 

 ate from each other and joined twins were formed. 



Driesch,(6) while working with the eggs of sea urchins, found 

 that if the fertilized eggs were subjected to a constant temper- 

 ature of 31 °C. for eight hours the eggs would begin to develop. 

 The cells would separate partially or completely from each other, 

 each part of the divided egg developing into an embryo. If 

 the cleavage was incomplete, joined twins were formed. Not 

 only were twins produced, but in several instances the blasto- 

 meres separated at the four-cell stage and four embryos devel- 

 oped. More than four divisions were never observed. These 

 experiments conclusively prove that two or more embryos may 

 develop from one ovum. 



Loeb,(7) likewise, produced twins experimentally with the egg 

 of the sea urchin, but employed an entirely different method 

 than that of Driesch. Fresh, fertilized eggs were put in an 

 equal mixture of sea water and fresh water. Due to the differ- 

 ence of osmotic pressure, the eggs absorbed water so rapidly 

 that the cell membrane ruptured and part of the protoplasm 

 extruded. After the membrane had ruptured, the eggs were 

 immediately placed in sea water and segmentation began. Cells 

 wandered into the extruded protoplasm, and the development 

 of twin embryos began. If the extruded protoplasm adhered 

 firmly to the protoplasm within the cell, joined twins were 

 formed, but if it became separated, two single embroys, or twins, 

 were formed. 



Wilson (8) was the first to extend this work to vertebrate eggs. 

 While experimenting with the eggs of Amphioxus, he found 

 that, by gently shaking the fertilized eggs in a solution, the 

 blastomeres were partly or completely separated at the two-, 



