TRANSLATORS INTRODUCTION xvii 



In these, now, as in the marine ancestors of all the 

 vertebrates, the fertilised egg is a tiny cell provided 

 with very little yolk, and set adrift in the sea- 

 water. The first division of the nucleus, and each 

 subsequent division of the daughter-nuclei, is at 

 once followed by division or segmentation of the 

 whole cell. The plane between the two cells thus 

 formed is called the first cleavage-plane, and is 

 regarded as vertical. The second cleavage-plane 

 is at right angles to the first, and is also vertical, so 

 that the little embryo consists of four cells, all on 

 the same horizontal plane. The third cleavage- 

 plane is horizontal, and divides the four cells into 

 an upper and lower tier of four cells. In the course 

 of a series of divisions the eight cells come to form 

 a hollow sphere the blastosphere enclosing a 

 cavity known as the cleavage or segmentation 

 cavity. 



The first great modelling then occurs. At one 

 side the single layer of cells, of which the wall of 

 the blastosphere is composed, begins to bend 

 inwards, just as a dimple forms in a hollow india- 

 rubber ball if a pin-prick allow some of the con- 

 tained air to escape. Further cell-divisions occur, 

 and the invagination becomes deeper, until the in- 

 vaginating wall nearly touches the wall which has 

 retained its primitive position. The embryo has 

 thus become a hollow cup, the walls of which are 

 double. The cup elongates, and its mouth, origin- 

 ally wide open, becomes more and more narrow, 

 until it forms a small pore opening into an elongated 

 blind sack. The embryo in this stage is known as 



