MOVEMENTS AND FOLDINGS 55 



rough sketch of their gastrulation in the space 

 available, because different members of the group 

 have evolved quite different ways of developing. 

 Probably the extreme diversity of the processes of 

 gastrulation in mammals is due to the difficulties 

 raised by the fact, so advantageous in many ways, 

 that the embryo remains in the maternal womb. 

 There are actually two main problems which arise. 

 In the first place, although the mammals are derived 

 from reptiles whose eggs contain a large quantity of 

 yolk, they themselves produce eggs which are 

 extremely poorly provided with yolk, all the necessary 

 nutriment being derived from the mother. Secondly, 

 a whole set of organs have to be developed to make 

 an adequate connection with the wall of the womb. 

 The mammal, having evolved through a stage with 

 yolky eggs, recapitulates this stage in its develop- 

 ment, in accordance with Haeckel's recapitulation 

 hypothesis, but it converts the organs which in its 

 ancestors were used to absorb the yolk into the 

 placenta and other organs by which it is attached 

 to the mother. The placenta is porous, and allows 

 oxygen and food substances to diffuse from the 

 maternal blood into the embryonic blood, which is 

 carried along the blood-vessels in the umbilical cord 

 into the foetus, which is thereby nourished and 

 sustained. 



The mammalian embryo, again like its ancestors 

 the reptiles, goes through a primitive streak stage, 

 but as the tgg contains very little yolk it arrives at 

 this stage in rather a different way. The cleavages 



