16.7.2 Sexual reproduction
The sexual reproduction is based on the fusion of two haploid cells (gametes) to form a diploid zygote. In multi-cellular animals the sexual reproduction is based on meiosis, which results in the formation of specialized sex cells called gametes. The two gametes from two parents, or from one parent (in the case of self fertilization) unite together to form zygote which is the first diploid cell of the new generation.
Fig 16.16 (a) Spermatogenesis
The gametes are of two types: male gametes or sperms and female gametes or eggs. The male gametes are formed in testes while the female gametes are formed in ovaries. Testes and ovaries are known as male and female gonads respectively. The production of sperms is called spermatogenesis while production of ova or eggs is called oogenesis (fig 16.16). Gametes form the physical link between the parent generation and the offspring.
The meiosis plays the fundamental roles in sexual reproduction. It provides the mechanism for reducing the number of chromosomes to one half in gametes and for the exchange of genetic material in chromosomes by the phenomenon of crossing over. The diploid number of chromosomes in the zygote provides the genetic make up for the new individual. The zygote, by simple mitosis changes into embryo, and the embryo develops into a new individual by cell division, differentiation and growth.
Fig 16.16 (b) Oogenesis
Table 16.2 comparison of asexual reproduction with sexual reproduction
| Asexual Reproduction | Sexual Reproduction | |
| 1 | Only one parent in involved | Two Parents, take part |
| 2 | No meiosis, No gamete formation | Meiosis takes place and gametes are formed. |
| 3 | No fertilization | Fertilization takes place |
| 4 | No variations, offspring are identical to their parents | |
| 5 | Rapid production of large number offspring | Less rapid increase in number |
16.7.1 Reproduction in grog
A fertilized egg or zygote of frog passes through a series of events to form an adult animal. The process involving these events is known as development. The developing individual in its early stage is called an embryo and in placental mammals at the later stage it is called a foetus. The study of embryo is called embryology.
Fertilization
The eggs are produced in paired thin walled sacs called ovaries located in the abdomen. Each egg is a small ball spherical structure having one half pigmented, called animal hemispheres, the other half yellowish due to the storage of food or yolk, and is called vegetal hemisphere. The middle of these two halves is called equator. The animal hemisphere or the upper half contains the nucleus of the egg cell. After the sperm enters the egg a small grayish crescent shaped area called grey crescent is formed on one side of the animal hemisphere close to the equator.
The male sex cells or sperms are produced in a pair of testes, during their breeding season. Female releases eggs and male release sperms in water, where fertilization takes place this is called external fertilization. Fertilization results in the formation of zygote.