Cleavage, or initial cell division, of the fish zygote is meroblastic, meaning the early cell divisions are not complete. The yolky end of the egg (the vegetal pole) remains homogenous while the other end (the animal pole) undergoes cell division.
The fate of each of these first cells, called blastomeres, is determined by its location. This contrasts with the situation in some other animals, such as mammals, in which each blastomere can develop into any part of the organism.
Neurulation, the formation of the central nervous system, is different in fishes than in most other chordates. The neural tube begins as a solid cord formed from the ectoderm. This cord then sinks into the embryo and becomes hollow, forming the neural tube. This process contrasts with the process in other chordates, which occurs by an infolding of the ectoderm to form a hollow tube.
Many species of fishes are hermaphrodites. Some, such as the painted comber (Serranus scriba), are synchronous hermpahrodites. These fish have both ovaries and testes and can produce both eggs and sperm at the same time. Others are sequential hermaphrodites. These fishes start life as one sex and undergo a genetically programmed sex change at some point during development. Their gonads have both ovarian and testicular tissues, with one type of tissue predominant while the fish belongs to the corresponding gender.
- Gilbert 1994, pp. 185-187.
- Gilbert 1994, p. 247 (note).
- Gilbert 1994, p. 781.
- Gilbert, Scott F. (1994). Developmental Biology (4th ed.). Sunderland, Massachusetts: Sinauer Associates. ISBN 0-87893-249-6.