230 PRINCIPLES OF EMBRYOLOGY 



Quite a number of studies have been made on the epigenetic mechan- 

 isms which bring about the development of the teleost (Review: Oppen- 

 heimer 1947). They have dealt with several different species, none of 

 which has as yet been fully investigated and the results do more to whet 

 one's appetite than to provide a comprehensive account of the causal 

 embryology of the whole group. 



The earliest stage at which operations have been made is immediately 

 after fertilisation. Such work goes back to the experiments of Morgan in 

 the 1890s. The results of the most recent workers on Fundulus suggest that 

 any one of the first two, or of the first four, blastomeres can give rise to a 

 complete embryo. For example, Nicholas and Oppenheimer (1942) ob- 

 served embryo formation in sixty-five cases out of seventy-two in wliich 

 they had eliminated one of the first two blastomeres chosen at random. 

 There seems to be little sign at this stage of an organisation centre which 

 is localised as is the amphibian grey crescent. The existence of such a centre 

 is, however, claimed by Tung and Tung (1944), using the eggs of the gold- 

 fish Carassius. They removed part of the uncleaved egg either by cutting 

 it away with a knife or by pricking the future embryonic area and squeez- 

 ing some of the cytoplasm out through the hole. They found that the 

 earlier the operation was made after fertilisation, and the larger the part 

 of the normal protoplasmic region remaining, the better was the differenti- 

 ation. Fully normal embryos could be formed when the operation took 

 place in about the first quarter of an hour after fertilisation and at least a 

 half, or preferably more, of the protoplasmic region was left intact. In 

 the abnormal and reduced embryos, mesodermal tissues sometimes differ- 

 entiated rather well, but neural tissue does not appear to have been seen 

 in the absence of accompanying mesoderm. 



The same authors also divided the early blastoderm into two parts, cut- 

 ting along the cleavage planes in either the 2-, 4- or 8-cell stages. They 

 found that each part might sometimes give rise to a complete and normal 

 embryo. In other cases there was one normal embryo and one mass of 

 cells with no histological differentiation ; and in some cases both portions 

 gave this undifferentiated result. Two complete embryos can also be 

 fused together, in stages up to the i6-cell stage. They sometimes produced 

 complete, separate, twins, but occasionally one well-formed but oversized 

 embryo to which it was clear that both eggs had contributed. All these 

 results are exceedingly similar to those which have been found in the 

 Amphibia, and might indicate a similar underlying mechanism, namely 

 an organisation centre which becomes located in the plane of bilateral 

 symmetry soon after fertihsation, which is essential for the development 

 of the axial structures of the embryo, and which is capable of considerable 



