EMBRYO FORMATION IN OTHER GROUPS OF VERTEBRATES 239 



half blastomeres when the first cleavage furrow does not divide the grey 

 crescent region into equal parts. One may conclude that the rabbit egg 

 (and presumably that of other mammals) is certainly not of the mosaic 

 type, but may contain a localised organisation centre comparable to the 

 amphibian grey crescent; this would probably be related to the bilateral 

 structures described by Dalcq. (For experiments involving the alteration 

 of chromosome numbers, induction of polyploidy etc. in mammals, 

 see Beatty 1955.) 



Figure 11.8 



Three stages in the early development of the rat. Upper row, fertiHsed egg 

 with pronuclei; middle row, four-cell; lower row, young blastocyst with 

 inner cell mass. The eggs in the left column have been stained to exhibit 

 alkaline phosphatase; those in the centre column RNA; and those in the 

 right column mucopolysaccharides. (After original drawings of Dalcq.) 



A few experiments have also been made at the primitive streak stage 

 of rabbits although the material is technically very difficult to handle 

 owing to its stickiness, transparency and resistance to cutting. Wadding- 

 ton (1937), failed to get any inductions by fragments of primitive streak 

 transplanted between the epiblast and hypoblast in the manner used in the 

 chick, but the material was certainly not extensive enough for this nega- 

 tive result to have much importance. On the other hand, the competence 



