GENERAL BIOLOGY 



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 



Gl S~~*\ doubtless, when the two cells remain 

 c I ) 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 each 

 of the cells of the developing lancelet, 

 isolated at the 4-cell stage is capa- 

 ble of forming an embryo, but at the 

 FIG 208 The de- g - ce11 stage, each cell may develop 

 only ^ far as the blastula. Apparently 

 differentiation is slight at first, and 

 "ontogeny assumes more and more the 

 &o e iatldas e at s 7* character of a mosaic work as it goes 



two half embryos in 

 the i6-cell stage; e, 



?y m c| n in divid a ed 



thl 



Some aberrancies of regeneration. 



Ordinarily after mutilation, if normal 

 no?ml?size. tofhal 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 



