Biology of Finfish and Shellfish (2+1)

4.7.1.1. Embryonic development

In a fully ripe egg, a small opening known as micropyle appears in the shell. Through this micropyle, polar body escape and water from outside enters into it. This causes swelling of the egg. The swelling may be as much as four times of the original size. Now, a gap called perivitelline space is found in which the embryo is bathed during its development. Usually, the water mixes with the yolk and makes the egg to become transparent. It may be mentioned here that the fertilized egg is transparent whereas unfertilized one is opaque. After fertilization, the micropyle is closed and no more spermatozoa and even water can pass through. Gaseous exchanges, however, can take place through the vitelline membrane. The environmental conditions particularly the temperature and pH of water has greater influence on the developing embryo.

Embryonic development begins from the moment the egg is penetrated by a sperm i.e. just after fertilization. The embryonic development in fishes is basically the same as in other chordates. The fertilized egg first undergoes segmentation and thus it passes from one-celled to many celled stage. This segmentation is known as cleavage. It is the process by which the fertilized egg is divided into smaller cells called blastomeres. In both cartilagenous and bony fishes, the cleavage is incomplete and is confined to the superficial cytoplasmic layer. The deeper yolky portion remains unchanged (unsegmented). This type of cleavage were only a small disc like part (germinal disc) of the egg which is known as meroblastic and the disc of cells thus formed on the upper or animal pole is known as the blastoderm. Cleavage ultimately result in the formation stage which is characterized by the single layered cells (blastomeres) and having segmentation cavity (called blastocoels) formed under the blastoderm. A large number of free blastomeres form a layer of cells called the periblast and lie just above the yolk. Actually, the space between the blastoderm and periblast is the blastocoels. The blastoderm ultimately gives rise to future embryo.

In most bony fishes, gastrulation starts with the presumptive endodermal and mesodermal cells at the posterior end migrate forwards under the blastoderm, thus forming the hypoblast. The cell of the blastoderm has continued to grow over the yolk. This process is known as epiboly . The presumptive ectoderm cells grow over and cover the yolk mass from outside, forming a layer of cells called epiblast. Simultaneously periblast also grows and forms an inner covering of the yolk. The periblast and epiblast enclose the yolk in a yolk sac. Formation of yolk sac signifies the termination of gastrulation. The embryo proper is now distinctly separated from the yolk sac which can be seen from outside. The embryo is connected with yolk sac by a yolk stalk. Blood vessels develop in the wall of the yolk sac and as the embryo grows the yolk sac is gradually reduced in size. This indicates that the yolk sac provides nourishment to the developing embryo.

The characteristic feature of gastrulation in fishes is the formation of primary rudimentary organs which starts in the anterior part of the embryo. Various organs of the body are formed from the ectoderm, mesoderm and endoderm. The ectoderm gives rise to the epidermis and its derivatives like brain and spinal cord, the lens of the eye and internal ear. Similarly, muscles, appendages, axial skeleton, skin, scales etc. develop from mesoderm cells. Endoderm cells make up the inner lining of the digestive tract and sex cells. Certain endocrine glands such as thyroid and ultimobranchial glands are also derived from endoderm cells.

The embryonic phase is the interval in which the major organ systems begin to appear. It ends in hatching. However, the exact state of development into an embryo at the time of hatching not only varies among species but may vary within a species, depending on environmental conditions. The summary of the embryonic development in Clarias batrachus is given below .

Stage 1.	Fertilized egg.
Stage 2.	Two-celled stage (45 min. after fertilization)
Stage 3.	Four-celled stage (1 hour)
Stage 4.	Eight-celled stage (1hr 20 min.)
Stage 5.	Multi-celled stage (2 hrs. 30 min.)
Stage 6.	Morula stage (3 hrs. 30 min.)
Stage 7.	Formation of germinal ring (5 hours)
Stage 8.	Embryo formation (7hrs. 30 min.)
Stage 9.	Differentiation of head and tail ends of embryo (10 hours)
Stage 10.	Somite differentiate (12 hours)
Stage 11.	Formation of optic cups, eight somite stage (14 hrs. 30min.)
Stage 12.	12 somite stage (16 hours)
Stage 13.	Kupfer’s vesicle appears (18 hours)
Stage 14.	25 somite stage (19 hours)
Stage 15.	Optic cups are visible; Kupfer’s vesicle disappears (20 hours)
Stage 16.	Twitching movements starts (20 hrs 30 min.)
Stage 17.	Twitching movements more frequent; tail end gets freed (21 hours)
Stage 18.	Over 40 somite stage (21 hrs. 30min.)
Stage 19.	Twitching movements more vigorous, egg capsule weakens (21hrs. 45min.)
Stage 20.	Egg-capsule ruptures (21 hrs. 50min)
Stage 21.	Larva hatches out (21hrs. 55min.)