2 24 BLEMEWTS OF OENITEOLOGT. 



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 pelludda, 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. 



GrraduaUy 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 sliell, 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 tlie yelk is actually transformed 

 directly into the developing embryo — namelv, 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 epi- 

 hlast, 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. 



