94 CLEAVAGE AND DIFFERENTIATION 



it possible to separate the blastomeres of the frog at the 2-cell stage, 

 and it has been found that each blastomere thus isolated can (pro- 

 vided that it contains a portion of the grey crescent region) develop 

 into a whole embryo.^ 



It will be remembered, as explained in Chap, iii, that grafting 

 experiments have shown in the newt that the various regions 

 (except that of the organiser) are plastic up to a certain stage in 

 gastrulation, and that tissue which was presumptive epidermis can 

 differentiate into part of the brain and eye. Recent improvements 

 in technique have permitted of analogous experiments on anuran 

 material, and it has been found that the tissues of the frog (again 

 with the exception of the organiser) are plastic up to a similar stage.^ 



Cleavage of the egg of Anura, then, does not result in the 

 separation of qualitatively unequal cytoplasm between the blasto- 

 meres, certainly of the 2-cell stage, and presumably of the 4-cell 

 stage, with the exception of the specialised region of the grey 

 crescent. In this respect, therefore, the anuran egg does not differ 

 from that of the Urodele. 



A few more words may be added concerning the cause of the 

 production of the double monsters from embryos of the frog which 

 have been inverted at the 2-cell stage. It has been found possible 

 to obtain such monsters by inverting the undivided egg, and there- 

 fore the duplicity of the monsters is not due to the number of 

 blastomeres into which the egg has cleaved when it is inverted. 

 Triple monsters may also arise from inversion. These anomalies 

 have been shown to be due to the fact that when the streaming of 

 the yolk takes place, consequent on the inversion, a streak of inert 



^ Schmidt, 1930, 1933. 



2 Schotte, 1930; Schmidt, 1930. 



The question of the existence at early stages (fertilised but unsegmented egg) 

 of anuran development of cytoplasmic regions possessing a determination has 

 been attacked by the method of making small injuries with a heated or unheated 

 needle. Loss of tissue (by damage in situ or by extra-ovation) at this early 

 stage leads to the development of imperfect larvae, and it has been held that even 

 the unsegmented egg possesses (labile) determinations (Brachet, 1905, 1906, 191 1, 

 1923, 1927 ; Pasteels, 1932). But it is necessary to point out that the eventual mal- 

 formation or non-appearance of an organ after injury to the egg is, by itself, no 

 logical justification for the view that the rudiment of the organ in question was 

 determined at the stage operated upon: the injury done to a particular part of an 

 egg persists, and may exert an inhibiting influence on the subsequent determination 

 and chemo-differentiation of whatever rudiment comes to occupy its site. On the 

 other hand, clean removal of pieces of blastulae (other than the organiser-region) 

 in Bombinator and Triton allows normal development to occur (Bruns, 193 1). 



