Cleavage and formation of periblast
In the fertilized egg of teleosts cytoplasm is accumulated at the animal pole as the blastodisc1, which houses the nucleus2. This was first described by ‘Coste (1850)’. In some teleostean species the streaming of cytoplasm towards the blastodisc is said to continue after fertilization, e.g. in Gasterosteus pungitius ‘(Ransom, 1867)’, the pike ‘(Truman, 1869)’, Salmo fario ‘(Oellacher, 1872)’, Gadus morrhua, salmonoids, clupeoids and cyprinodonts ‘(Ryder, 1884)’, Crenilabrus rostratus, C. pavo and Tinca vulgaris ‘(Janosik, 1885)’, Salmo salar, Cyprinus carpio, Gadus morrhua and Rhodeus amarus ‘(Reis, 1910)’, Brachydanio rerio ‘(Thomas and Waterman, 1978)’. This ‘late arriving cytoplasm’ contributes also to the thickened edge, the so-called annulus3, as has been shown by the movements of injected chalk particles ‘(Long, 1980)’. The blastodisc is facing downwards in most pelagic and upwards in demersal eggs. At the edge the blastodisc diminishes into a very thin layer of cytoplasm (YCL) which surrounds the whole of the yolk4.
KeywordsCell Stage Nuclear Division Marginal Cell Surface View Animal Pole
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