358 



GENERAL BIOLOGY 



a 



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 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- ^-ccll Stage, cach ccll may develop 

 emb?>"irin*divided ^^^Y ^^ far as the blastula. Apparently 

 (ffter'^Morgan)^^? differentiation is slight at first, and 

 the egg. 6 the same "ontog^env assumcs morc and more the 



when It was divided: o J 



iJSatld as^at l^^d character of a mosaic work as it goes 



two half embryos in fr-i-r-in^o-rr! " 

 the i6-cell stage; e, -Or^\a^a. 



pktf'b?aSuirs°'/^ Some aberrancies of regeneration.— 



the gaSmil^^Tage" Ordinarily after mutilation, if normal 

 no?maUizr°^^^^^ couditions bc 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 



