323 



HEREDITY AND ENVIRONMENT 



If the cleavage cells are only partially sep- 

 arated they may produce animals which are 

 partially separated, such as Siamese twins, 

 two-headed forms, etc. (Fig. 71). Or these 

 double monsters may be produced by division 

 or budding of the embryo at a later stage of 

 development. In the human species, no less 

 than in other animals, all sorts of double mon- 

 sters may be formed in this way by the partial 

 division of a single egg or embryo (Fig. 76). 

 If the division is slight the developed indi- 

 vidual may show only the beginnings of a divi- 



FiG. 75. Two larvae of Styela which were centrifuged in 

 the 4-celled stage thereby changing the position of various 

 organ-forming substances. Nervous system (ms), eyes (S), 

 notochord {ch) and muscles {ms) have been displaced, and 

 the larva has been turned inside out, the endoderm {end) 

 being outside and the ectoderm (eci) inside. 



