DEVELOPEMENT. 
67 
discover that this germ is a disc composed of two laminae 
or layers, one lying over the other, the whole acting as a 
cover to the orifice of the tube leading to the cavity 
in the centre of the yolk. These laminae show themselves 
to be composite structures, consisting of two cohesive 
masses of very minute globules. After a short period of 
incubation one may perceive that these globules have 
become more numerous, owing to subdivision, and 
taking the form of cells or bladders, filled with fluid, 
become one with the floating germ. This is the first 
evidence of the working of life in an egg. 
Some hours later the cells show an increase in number 
as well as in size, and the lower layer of the disc of the 
germ becomes divided into two parts, forming two other 
laminae. At the fifteenth or sixteenth hour of incubation 
a fine streak of irregular thickness arises from the centre 
of the upper layer of the disc, taking a direction parallel 
to the shorter axis of the egg: this is the origin of the 
vertebral column. The germ-disc has, during this time, 
thickened and expanded, and its two upper laminae 
have become blended, taking the same direction as the 
above-mentioned streak, and therefrom proceed to the 
so-called proto-vertebrae. In the course of the next six 
or eight hours the edges along the sides of the streak, 
becoming raised, approach each other, and, uniting after 
contact, now form a hollow tube. At the same time there 
appears on either side of the same small cubes or dice; 
these form later the vertebrae. The upper ones among 
these are soon distinguishable by a bladder-like enlarge¬ 
ment which proceeds from them: this is transformed 
into the skull. The separate layers of the germ-disc 
have become extended, the upper one spreading over the 
superior surface of the yolk, the middle one exactly 
