28 



CLEAVAGE AND DIFFERENTIATION 



The relative unimportance from the point of view of differentia- 

 tion of the way in which the egg cleaves is revealed by the following 

 experiments. 



When a blastomere of a sea-urchin is isolated at the 2- or 4-cell 

 stage, it develops, as already mentioned, into a whole larva, but the 



6d)w.H 



Fig. 60 



Regulation in the insect egg. a. Normal embryo of the dragon-fly Platycnemis 

 pennipes, seen from the left side, b, Dwarf embryo, obtained by partial constric- 

 tion of the egg at the 4-nucleus stage ; the dwarf is normally proportioned and 

 developed and its organs have arisen from regions the presumptive fate of which 

 was quite different ; their fates were therefore not irreversibly determined at the 

 stage operated upon, and regulation has been possible. At. antenna; An. eye; 

 Epf. hindgut; M. mandible; M.Ch. chitinous muscle-attachments; Md. midgut; 

 M.Vi, first maxilla; Mx.^, second maxilla; O. labrum; Pr. proctodaeum; Sch.Ch. 

 apical chitin; St. stomodaeum; Stg. spiracle; Thi_^, first to third thoracic legs; 

 Schw.K. gills. (From Seidel, Biol. Zentralbl. xlix, 1929.) 



cleavage which it undergoes is the same as that which it would have 

 undergone if it had been left in contact with its sister-blastomeres. 

 In normal development in these forms, the first and second cleavages 

 are meridional and equal : the third cleavage is latitudinal and equal ; 

 the fourth cleavage in the animal hemisphere is meridional and 

 equal, in the vegetative hemisphere it is latitudinal and unequal. 

 Each cleavage division is therefore recognisably distinct. Now, in 



