Development of a chick was the difficult most thing in my study of Biology. In the whole chapter of Growth and Development, I paid a lot of time in understanding this topic. The most difficult part is the Gastrulation and, in it, the Notochord formation and Neurulation. Unfortunately, in the book of FSc II, there are only the cross-sectional pictures of Gastrula. But thanks to Internet and YouTube that I could see every available view. Those videos are attached at the end of the post. Whenever I find difficulty in any of my topic, I switch to YouTube or Wikipedia but in Biology, I use Wikipedia only for definitions because it gives a lengthy explanation that is not required for our level. So, start with our topic…
First of all, the definition of Embryology, as given in the Biology book of FSc II, “Embryology is the study of growth and differentiation undergone by an organism in the course of its development from a single fertilized egg into highly complex and an independent living being like his parents.”
I don’t know why we study the development of chick but they say that the early differentiation of the organ systems and the fundamental process of body formation is common in all vertebrates.
Fertilization and Incubation
First of all comes the ‘Fertilization and Incubation’. In chick, the fertilization is internal and the shell is secreted as the egg passes the shell gland. Egg is laid and development stops until the appropriate temperature (nearly up to the body of mother) is provided. When the egg is hatched in an incubator, the temperature is regulated between 36-38 degrees Celsius.
Then comes the stage of cleavage. The picture provided in our book has only the dorsal view but thanks to Sir Inam-ur-Rehman Bhatti that he showed us other views which made it easy for us to understand cleavage.A series of mitotic divisions is called cleavage. The cleavage of a chick is called discoidal cleavage (“the process of cell division is confined to the small disc of protoplasm lying on the surface of the yolk at the animal pole”) The first two cleavage planes are vertical, cutting each other, while the third one is horizontal that cuts underneath the cytoplasm and it gets separated from yolk. The first two cleavage planes could be easily understood by the dorsal view but not the third cleavage plane.
Then comes morulla stage. A morulla is a “rounded closely packed mass of blastomeres.” This stage is short-lived. Morulla consists of two or more layers of disc-shaped mass of cells called blastoderm. The cells are smaller and completely difined at the center of the blastoderm while cells are larger and flattened at the corners of blastoderm.
The stage of blastula comes after the stage of morulla. Blastula is characterized by the presence of a segmentation cavity called blastocoel. Blastula consists of blastoderm, blastocoel and yolk. The corners or the marginal area of the blastoderm is called the zone of junction. The zone of junction remain attached with yolk.
Now, Gastrulation which was very difficult for me after the formation of primitive streak means when it used to approach at the formation of notochord. In this stage, the blastoderm gets separated in two layers. The upper layer, epiblast consists of presumptive ectoderm and presumptive mesoderm while the lower layer, hypoblast consists of presumptive endoderm. The hypoblast cells grow outward over the surface of the yolk, then downwards around it to form the endodermal lining of a yolk sac.
Area Pellucida and Opaca
At gastrulation stage, blastoderm gets separated from the yolk. The central part of blastoderm rise due to the fluid development. This gives rise to a translucent area called area pellucida from which the light can not pass. This is the yellow part of the yolk. While, the marginal area or peripheral area (or you can say the corners of blastoderm which were termed as zone of junction) remained attached to the yolk and form the more viscous area called area opaca because this area transmits the light. This is the white part of the yolk.
Formation of Primitive Streak, Primitive Groove, Primitive Ridges and Hensen’s Node
In chick, the mesodermal cells migrate medially and caudally to form a mid line thickening called primitive streak. Presumptive mesodermal cells continue migrating and the length of primitive streak grows and finally, the shape of blastoderm changes from circular to pear. The primitive streak elongates almost half of the length of ectoderm. The anterior end of the primitive streak is occupied by an aggregation, the primitive node or notochordal cells while the rest of the cells are mesodermal cells. The cells continue to migrate between epiblast and hypoblast and form mountain like ridges called primitive ridges and the groove between those ridges is called primitive groove. Primitive ridges and primitive groove are formed from primitive streak. Primitive node is called Hensen’s node, in birds, which is a thickening at the top end of primitive streak.
The difference between primitive node and Hensen’s node was the difficult thing for me as neither there was any picture of Hensen’s node or primitive node in our book nor they showed any relation between them. Although Wikipedia helped me but it couldn’t explain it as it is an encyclopedia. The website that helped me (and I liked it) while searching this was
Formation of Notochord, Primitive Gut and Somites
After this, the cells start pushing in from the region of Hensen’s node and form a rod like structure, beneath the ectoderm, called notochord. Notochord is a prominent feature in a chick embryo of 18 hours. The ectoderm becomes a coherent layer of cells merging with yolk and marginal area and forms germ wall, where the expanding germ layers merge with underlying yolk. The cavity between the yolk and the endoderm, which was previously called gastrocoele is now termed as primitive gut. Hensen’s node form the dorsal mesoderm which form the somites. The groove between them is called neural groove. Remember that primitive groove and ridges were in primitive streak (in almost one half) while neural groove and somites are formed above the notochord (in almost other half). Mesoderm gets split in somatic mesoderm and splanchnic mesoderm and the space between them is called coelom. Somites are seen in 25-26 hours embryo.
Above the notochord, presumptive ectoderm is present in the form of a band which thickens with the elongation of gastrula and forms neural plate. In an embryo of 18 hours, the neural plate is just seen as a flat and thickened area. In the embryo of 21-22 hours, the neural groove is seen while in the embryo of 24 hours, the neural plates are visible as the folding on both sides of neural groove. At this stage, the embryo is called neurula. The anterior end of the neural groove is widest and makes brain while the rest of makes spinal chord. On the other hand, neural folds move towards each other and unite to make a tube neural tube. The neural groove converts into neural tube while the neural plates separates and makes a layer which will form skin. The anterior and the posterior openings of the neural tube are called neuro-pores which close later on and the cavity enclosed is known as neurocoel.
Neurulation is difficult until or unless you can not differentiate between primitive groove, neural groove, ridges and somites. Unfortunately, in the book of FSc II, there are only three cross-sectional pictures which gives an image that all process is happening at one end.