304 BOARD OF AGRICULTURE. 



at one end, with indentations right and left, and then the mar- 

 gins of the disc spread, and, folding downward, enclose more 

 and more of the yolk, and the sides of the furrow thicken, 

 so that represented in profile it will be no longer a shallow 

 furrow, but something like a channel or tube. 



At this stage the whole mass has still about the same con- 

 sistency everywhere. It is like soft jelly and a little pulpy, 

 but presently the two edges of the furrow come more closely 

 together, and finally touch. Meanwhile the margins of the 

 new being rise in a fold and enclose the central parts, forming 

 a sac around the germ known as the amnios. The natural re- 

 sult of the closing of the upturned edges of the germ is the 

 formation of a cavity, enclosed between these edges. That 

 cavity now fills with a transparent fluid, and as it fills there 

 appears something a little more substantial upon its sides and 

 below it ; the walls protecting the cavity become less transpar- 

 ent, or even slightly opaque ; then the cavity widens sideways 

 on its anterior part, and rises a little from the rest. In one 

 word, this cavity forms the channel for the spinal marrow, and 

 its front part the cavity for the brain, and the walls grow to be 

 flesh and bone to form the dorsal spine. The upper part 

 represents the axis of the skeleton, with the surrounding soft 

 parts ; the lateral parts form the ribs with their fleshy cov- 

 ering, and the animal thus closing over the yolk, we have the 

 abdominal cavity. Now, it requires a little more enlargement, 

 a little more change into d.flerent substances, to complete 

 the formation of the new. The gelatinous substance outside 

 the main axis is changed into a fibrous structure, which is 

 muscle. The little opaque bodies in the axis and upon its 

 sides absorb some earthy material contained in the primitive 

 substance from which they have arisen, and thus bone is 

 formed. The fluid in the upper cavity becomes a little more 

 granular and more solid, and it is the brain and spinal marrow. 

 The yolk is absorbed during the process of growth, but the 

 wall within which it is contained is elongated and enlarged, 

 and in consequence of further changes in the substance of that 

 part of the yolk which is in immediate contact with the body- 

 walls, the alimentary cavity is formed. You have, in fact, all 

 the organs of the animal growing in the same way, by suc- 

 cessive transformations of the homogeneous mass iuto ail the 



