224 ELEMENTS OF ORNITHOLOGY. 
statement of a few fundamental facts and a short account of 
certain structures the temporary distinctness of which enables 
us to understand the nature of parts which ultimately show no 
trace of their earlier divided condition. 
The ovary has been compared to a small bunch of grapes, 
but these grapes vary greatly as to size. They are, of course, 
the immature eggs or ova. The smallest consist of but a 
microscopic spheroid of the substance called protoplasm * en- 
closed in a delicate membrane, the zona pellucida, and containing 
within its substance a denser particle called the nucleus or 
germinal vesicle, within which again is a minute distinguishable 
particle, the nucleolus or germinal spot. 
Gradually one ovum after another increases till its protoplasm, 
the yelk, becomes of large size. It is enclosed within a mem- 
branous envelope, the ovisac, which ruptures and allows the 
ovum, when ripe, to escape into the upper, open end of the 
oviduct. As it descends this tube it becomes coated with an 
albuminous secretion, the white, and further down that tube it 
receives its calcareous investment, or shell, and very often layers 
of pigment according to the colours which may characterize the 
eggs of this or that kind of bird. 
But a very small portion of the yelk is actually transformed 
directly into the developing embryo—namely, a small patch on 
the surface familiarly known as the tread. The rest of the yelk 
serves but to nourish the embryo. 
From this small superficial patch of protoplasm all the varied 
tissues and all the complex parts which constitute the adult Bird 
are, by degrees, derived. The primitive cell of which the em- 
bryo, at the very first, consists, divides and subdivides itself 
again and again till three layers of cells are gradually but 
rapidly formed. The most superficial of these is called the epz- 
blast, the deepest the hypoblast, while between them is the meso- 
blast. 
Soon a slight longitudinal furrow is formed, called the primi- 
tive streak ; but much more important is another longitudinal 
groove (more anteriorly situated with respect to the embryo, as 
subsequent development shows), the medullary groove, wherein 
is laid the foundation of the brain and spinal cord ; while beneath 
* A term proposed by Mohl to denote the soft interior of cells. It is 
a soft, viscid, transparent, colourless substance, easily decomposable. It is 
resolvable into oxygen, hydrogen, nitrogen, and carbon, and with traces of 
some other chemical elements. 
