232 BOARD OF AGRICULTURE. 



yolk. As yet it is only yolk flowing through yolk, though 

 the flowing yolk is somewhat different in appearance from 

 that which surrounds it, and which' afterward builds up the 

 walls of the vessels. Meanwhile, the walls of the longitudinal 

 furrow, formed by the rising ridges along the middle line of 

 the germ, meet and unite, thus transforming the open furrow 

 into a close cavity. One end of this cavity becomes some- 

 what wider then the rest and has lateral undulations. Within 

 this portion of the cavity is hereafter to be the head. 



At some future time, long after the period I now describe, 

 there will be not only a brain forming in this front cavity, 

 but also all those parts which constitute the head and connect 

 with the spinal-marrow, which is enclosed in the straight and 

 narrow part of the cavity. The lower edge of the embryonic 

 disc goes on increasing, extending gradually downward over 

 the yolk until it has taken in the whole mass, closing over it 

 so as to form the walls of another cavity below ; this cavity, 

 though it is now full of yolk, and contains nothing else, will 

 in the course of time be transformed by a series of changes 

 into a mouth, lungs, chest, abdomen, in short all those or- 

 gans of respiration, digestion, etc., by which life is main- 

 tained. But as yet, I repeat, it is nothing but a bag full of 

 yolk, enclosed in a bag of yolk-cells, which have undergone 

 certain slight changes from their primitive character. The 

 upper cavity is nothing but a channel formed by the ridges 

 on the surfiice of the germ, which have become united along 

 the middle line. The channels of moving yolk arising from 

 a pool, here and there, are only gradual depressions in the 

 yolk-mass, slowly covering or enclosing themselves with yolk- 

 cells, or with a yolk of a peculiar kind, which forms walls 

 around them. Suppose that water flowing through a rut on 

 the ground, after a heavy rain, should line the Avails of that 

 rut with mud, so as to consolidate the rut, and that with 

 every rain this process should go on always increasing 

 aud strengthening the consolidation, you would then have a 

 process resembling that through Avhich these channels of fluid 

 yolk are transformed into vessels, into closed tubes, circum- 

 scribed by the coherence of minute particles of the fluid itself, 

 which adheres to the sides of the channels, and changes 

 what were at first mere ruts into the closed cylinders, aque- 



