220 THE MOSAIC STAGE OF DIFFERENTIATION 



its ability to rotate within its membranes and the greater weight of 

 its yolk cause it to orientate itself in the centrifuge tube in such a 

 way that the animal pole is centripetal and the vegetative centri- 

 fugal. The result of centrifugalisation is therefore an intensification 

 of the stratification normally found along the primary egg-axis. The 

 yolk is concentrated more densely than ever at the vegetative pole ; 

 above it is a layer of clear cytoplasm, and the animal pole is occu- 

 pied by a layer of fat. If the centrifugalisation is heavy, develop- 

 ment proceeds a certain way and then stops, largely owing to 

 mechanical difficulties arising from the inertia of the abnormally 

 dense mass of yolk. But if the centrifugalisation is light, develop- 

 ment is normal except for the fact that the structures of the head 

 contain an abnormally large amount of fat. The cells of the brain 

 may contain many times the normal quantity of fat, but neverthe- 

 less the differentiation of the brain and the development of its form 

 are normal. Similarly, some of the regions of the trunk can develop 

 normally although their cells contain less than the normal quantity 

 of yolk. It is obvious, therefore, that yolk and fat are only raw 

 materials. 1 When, however, the amount of fat at the animal pole 

 exceeds a certain proportion, normal development is impossible. 

 Vacuolisation is the first sign, but in more extreme cases the 

 brain and other head-structures are reduced to a small degenerated 

 mass of cells ^ (fig. 105). 



We may illustrate the part played by the yolk and fat in the frog's 

 egg with the help of an analogy. The construction of a conservatory 

 is of course conditioned by the availability of the necessary raw 

 materials — wood and glass. There is an optimum proportion in 

 which these materials should be present in order to give the best 

 results, but this proportion may be altered in either direction up to 



^ Jenkinson, 1915. 



- With slightly heavier centrifugaHsation, curious malformations appear in the 

 trunk region. The myotomes are frequently fused together beneath the nerve tube, 

 with consequent absence of the notochord. The spinal gangHa may also be fused 

 ventrally beneath the nerve tube. The latter has an abnormally thick floor and 

 thin roof, with the white matter concentrated ventrally. From other experiments 

 (see Chap, xi, p. 375), it is known that these effects are associated with notochord 

 absence, and it is probable therefore that here absence of the notochord is the 

 cause of the other observed changes, but the cause of this primary change re- 

 mains for the present obscure. Possibly the centrifugalisation has resulted in an 

 alteration in the composition of the organiser region : further research is needed 

 on the question. 



