MOVEMENTS AND FOLDINGS 59 



part way up the back. Both these sorts of monstrosi- 

 ties sometimes occur in man, if anything happens 

 to disturb development, but the embryo is so well 

 guarded from casual disturbances, well hidden as it 

 is in the womb, that they are fortunately rare. 



It is interesting to notice that mammalian embryos 

 are relatively very slow in arriving at the primitive 

 streak stage of development, but then run ahead 

 quickly till the main parts of the body are present, 

 slowing down again, in the case of man at least, for 

 the final elaboration of the parts before birth. Exact 

 comparisons of the number of days taken by different 

 embryos to arrive at comparable stages are not very 

 informative; what is important is not the absolute 

 times but the relative times for various amounts of 

 development within the same life-history. Thus a 

 newt takes about one day to get to the beginning of 

 gastrulation, about one day to gastrulate, and about 

 two days to develop from the end of gastrulation to 

 an embryo with fifteen body-segments (the times 

 actually depend on the temperature, since the 

 warmer the egg is kept the faster it develops; but 

 the temperature does not seriously affect the relation 

 between these lengths of time). A rabbit embryo 

 takes nearly eight days to get to the beginning of 

 gastrulation, and then like the newt about one day 

 to gastrulate and two to develop into an embryo 

 with fifteen body-segments. A human embryo takes 

 about a fortnight before arriving at the primitive 

 streak stage. This slowness in early stages, and speed 

 in later stages, is probably due to the fact that 



