Jl READINGS IN BIOLOGICAL SCIENCE 



M2oth of an inch in diameter. It contains a mass of viscid nutritive matter 

 — the yelk — within which is inclosed a second much more dehcate sphe- 

 roidal bag, called the gemiinal vesicle. In this, lastly, lies a more solid 

 rounded body, termed the germinal spot. 



The egg or Ovum, is originally formed within a gland, from which, in 

 due season, it becomes detached, and passes into the living chamber fitted 

 for its protection and maintenance during the protracted process of gesta- 

 tion. Here, when subjected to the required conditions, this minute and 

 apparently insignificant particle of living matter, becomes animated by a 

 new and mysterious activity. The germinal vesicle and spot cease to be 

 discernible (their precise fate being one of the yet unsolved problems of 

 embryology), but the yelk becomes circumferentially indented, as if an 

 invisible knife had been drawn round it, and thus appears divided into two 

 hemispheres. 



By the repetition of this process in various planes, these hemispheres be- 

 come subdivided, so that four segments are produced; 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 juicleiis. Nature, by 

 this process, has attained 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 arrangement, becoming converted into 

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

 appears a thickening, and, bv and by, in the centre of the area of thicken- 

 ing, a straight shallow groove 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 inclosed cavity dilates to form the head, 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 by, 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 accurately, and only, at last, receives the touches 

 which stamp its final character. 



Thus, at length, the young puppy assumes such a form. In this condi- 



