ii THE CELLULAR EMBRYO. 85 



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 thick- 

 ening, 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 posi- 

 tion 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 eventu- 

 ally lodge the spinal marrow and the brain; and 

 in the floor of this chamber appears a solid cellu- 

 lar 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 be- 

 comes 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, as- 

 sume the shape of limbs. Watching the fashion- 

 ing 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 



