[I THE CELLULAR EMBRYO 85 



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 arrangement, 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 

 marrow and the brain ; and in the floor of this 

 chamber appears a solid cellular cord, the so-called 

 notochord. One end of the enclosed cavity 

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

 remains narrow, and eventually becomes the tail ; 

 the side walls of the body are fashioned out of the 

 downward continuation 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 wero 



