An experimental analysis of the development of the haploid syndrome in embryos of Xenopus laevis.

H A P L O I D vertebrates may occur spontaneously but are very rare (Fankhauser, 1941; Humphrey & Fankhauser, 1957); however, haploids may be experimentally produced in fish (Swarup, 1959) and in mammals (Beatty, 1953), while amphibian eggs may be so treated that all developing embryos are haploid (Porter, 1939; Gurdon, 1960). The full descriptions of the development of haploid Rana pipiens (Porter, 1939) and R. nigromaculata (Miyada, 1960) apply so well to Xenopus laevis that only the most important points will be touched on here. Haploid amphibians may be identified at the beginning of gastrulation since their animal pole cells are smaller at a given stage than are those of diploids. In all haploid Anura the onset of gastrulation is delayed, and thereafter haploids become progressively more retarded in their development. Their neural plates are shorter, and when the neural folds have closed it can be seen that the embryos are microcephalic and suffer from lordosis and a bulging abdomen. These anomalies are common to all haploids; but after hatching it can be seen that the majority of haploids have a feeble heart and sluggish blood circulation, which may be the reason for the oedema to which the animals are subject. They are much less active than diploids and will lie at the bottom of the dish unless provoked into violent but short-lived activity. Their muscle fibres remain spindle-shaped and do not develop into compact blocks of tissue as do those of diploids. Whereas 90-95 per cent of haploid Xenopus develop the haploid syndrome as described, the remainder become normal looking tadpoles despite having shown the early signs of haploidy before hatching. Haploids of other Anura