330 



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 (ns), eyes (E), 

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

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

 being outside and the ectoderm (ect) inside. 



