TO THE LOWE.R ANIMALS. 77 



these hemispheres become subdivided, so that four seg- 

 ments are produced (D) ; and these, in like manner, divide 

 and subdivide again, until the whole yelk is converted 

 into a mass of granules, each of which consists of a minute 

 spheroid of yelk-substance, inclosing a central particle, the 

 so-called ' nucleus ' (F). [Nature, by this process, has at- 

 tained much the same result as that at which a human 

 artificer arrives by his operations in a brick field. She 

 takes the rough plastic material of the yelk and breaks it 

 up into well-shaped tolerably even-sized masses — handy 

 for building up into any part of the living edifice. 



Next, the mass of organic bricks, or ' cells ' as they are 

 technically called, thus formed, acquires an orderly ar- 

 rangement, becoming converted into a hollow spheroid 

 with double walls. Then, upon one side of this spheroid, 

 appears a thickening, and, by and bye, in the centre of 

 the area of thickening, a straight shallow groove (Fig. 14, 

 A) marks the central line of the edifice which is to be 

 raised, or, in other words, indicates the position of the 

 middle line of the body of the future dog. The substance 

 bounding the groove on each side next rises up into a fold, 

 the rudiment of the side wall of that long cavity, which 

 will eventually lodge the spinal mrrrow and the brain; 

 and in the floor of this chamber appears a solid cellular 

 cord, the so-called ' notochorcV One end of the inclosed 

 cavity dilates to form the head (Fig. 14, B), the other re- 

 mains narrow, and eventually becomes the tail ; the side 

 walls of the body are fashioned out of the downward con- 

 tinuation, of the walls of the groove ; and from them, by 

 and bye, grow out little buds which, by degrees, assume 

 the shape of limbs. Watching the fashioning process stage 

 by stage, one is forcibly reminded of the modeller in clay. 

 Every part, every organ, is at first, as it were, pinched up 

 rudely, and sketched out in the rough ; then shaped more 



