Fertilized 



egg cell 



First 

 segmentation 



(Possible twins) 



Second segmentation 



(Possible { 

 quadruplets) 



/ Third ^ 

 segmentation 



Yvonne 



Annette 



Ceciie 



Emilie 



Marie 



Kin^ FcMliiirs S,\iiclir;ile. Inc. 



FIVE GIRLS FROM ONE FERTILIZED EGG 



Each cell resulting from the first two or three cell divisions of a developing embryo 

 would seem capable of becoming a complete individual. The most reasonable inter- 

 pretation of the resemblances and differences among the "Quints" is that after the 

 second cell division, three of the four cells developed into Yvonne and Annette and 

 Ceciie, while the fourth cell divided again, developing Emilie and Marie 



Each of the cells resulting from the first three or four segmentations of 

 the egg is capable of developing into a complete individual. In some species 

 the capacity to form individuals is present in later cell-generations. By the 

 time the embryo has reached the two-layer stage (see page 351), each part 

 is fairly well set for its "destination". The end which is to become head is 

 already determined. The parts that are to form skin and nerves are already 

 distinct from the parts that are to form the food tube. Cells removed from 

 the outer layer, or ectoderm, can keep on growing in a suitable fluid. But 

 they will grow only ectoderm cells. Similarly, endoderm cells removed 



361 



