GENERAL BIOLOGY 



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in figure 208, producing two individuals of half the usual 

 size. At first they are likely to develop as half embryos, 

 each cell and its descendants behaving 

 as though the other were present. Con- 

 sequently the blastula when formed is 

 open on one side ; but it closes and forms 

 a normal embryo later. 



In most bilateral animals the first 

 cleavage plane lies in the medium 

 plane of the body that is to be, and 

 doubtless, when the two cells remain 

 together each develops its own half of 

 the body, left or right; but the above 

 experiment shows that either is capable 

 of developing any part of the body. 

 Frogs eggs, with one cell killed at the 

 two-cell stage, likewise develop at first 

 half embryos, which later become whole 

 ones. Wilson long ago showed that the 

 cells of the egg of the lancelet, iso- 

 lated at the 4-cell stage are each 

 capable of developing an embryo, but at 

 the 8-cell stage, each cell may develop 

 only as far as the blastula. Apparently 

 differentiation is slight at first, and 

 "ontogeny assumes more and more the 

 character of a mosaic work as it goes 

 forward." 



Some aberrancies of regeneration. 

 Ordinarily after mutilation, if normal 

 conditions be maintained, regeneration 

 tends toward the production of parts 

 like those removed. When the head is cut off a hydra it 

 produces a new head, and not a foot. What marked 



FIG. 208. The de- 

 velopment of half 

 embryos in divided 

 sea-urchin eggs 

 (after Morgan), a, 



is h o e i a tld as e at s 7 er d 



two half embryo's in 

 the i6-cell stage; e, 

 the same as incom- 

 plete blastulas; /, 

 the same later in 

 the gastrula stage, 

 complete but of half 

 normal size. 



