2 INTRODUCTORY I 



aim and scope of Experimental Embryology, and our duty in 

 these lectures will be to discover what progress recent research 

 has made towards the achievement of that ideal. 



In development three processes may be discerned, quite 

 distinct from one another but taking place concurrently. 

 These are growth, or increase of size, nuclear and cell-division, 

 and differentiation or, as Herbert Spencer defined it long ago. 

 increase of structure. 



A few well-known examples will serve to illustrate this. 



The unripe but full-grown ovum of the Sea-urchin (Strongy- 

 locentrotus lividus) is a minute spherical body, of a faint orange 

 colour due to the presence of a pigment at the surf ace (Fig. 1). 

 The large nucleus is excentrically placed 011 that side on which 

 there is a passage the micropyle in the external jelly mem- 

 brane. After maturation, the female pronucleus, much smaller 

 than the original nucleus, lies at first excentrically under the 

 micropyle, into which the polar bodies have been extruded, 

 but later wanders from this position. At the same time the 

 pigment is withdrawn from the larger part of the surface, and 

 concentrated in a superficial zone which is placed usually 

 though not always subequatorially, that is parallel to the 

 equator and nearer the vegetative pole. The equator is of 

 course the plane passing through the centre of the egg and at 

 right angles to the axis, while the axis is the line passing 

 through the micropyle and the centre of the egg, the micro- 

 pyle end of the axis being known as the animal, the opposite 

 as the vegetative pole. We shall have occasion to see later on 

 that this polarity of the ovum depends also upon the intimate 

 structure of the cytoplasm. 



The egg is now ready for fertilization. Shortly after 

 fertilization it begins to segment, and the planes of division 

 pass through the egg-substance in a perfectly definite and 

 regular way. The first division is meridional (including the 

 egg-axis), and therefore separates the egg into two equal 

 blastomeres, each of which has a similar share of the large 

 unpigmented region of the animal hemisphere, the pigment- 

 ring, and the smaller unpigmented region around the vege- 

 tative pole (provided, of course, the ring has its usual 



