Gene activity in early development / Eric H. Davidson.

  • Eric H. Davidson
Date:
[1968]
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Gene activity in early development / Eric H. Davidson. Source: Wellcome Collection.

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    THE SPECIES HYBRID EXPERIMENTS 15 the Hartwigs, and it is interesting to note that it was during the period in which he was associated with the laboratory of R. Hertwig that Boveri carried out his first hybrid merogone studies. Only a few years earlier the Hertwigs had described the formation of normal and merogonal sea urchin hybrids; the conceptual background of the new hne of investigation was almost as recent. At root in a theoretical (if not necessarily a historical) sense was the demonstration by Köheuter, who, as early as 1761, showed that male and female parents contribute equally to the hereditary characters of the offspring (35). Kölreuter's demonstrations apparently did not influence later workers in cellular embryology, and it was not until the writings of Nägeli in the 1880's that the attention of developmental biologists was drawn to this in¬ cisive early experimental study. By this time the conclusions Kölreuter had drawn were already assumed by many investigators; in the absence of the basic concept of equal parental contribution to inheritance it is of course impossible to understand the nature of pronuclear fusion and fertilization. Pronuclear fusion was apparently reported first by Warneke, who observed it in a snail egg in 1850, and by Biitschli (1873) who reported fusion in both nematode and snail eggs. Auerbach (1874) independently described pronuclear fusion in Ascaris, as did Hertwig and Fol in 1879 in the sea urchin [see Fol (36) for an extensive consideration of earlier and contem¬ porary references]. Shortly thereafter Strasburger described pronuclear fusion in plants. These observations were of the utmost significance in that they produced the conviction that the nuclei of the male and female gametes are in fact the vehicle in which are borne the parental hereditary determinants. Figure 3 (37) shows the pronuclei of a human egg as viewed in the electron microscope and also illustrates the apparent equality of the egg and sperm pronuclei, the very feature which was so suggestive to the early ob¬ servers. The true significance of the pronuclear fusion phenomenon did not become completely clear until 1883, with the publication of Van Beneden's careful observations of chromosomal movements before, during, and after fertilization in Ascaris. In Fig. 4 several of the essential plates from Van Beneden s classic paper (38) are reproduced. The use of Ascaris megalocephala contributed enor¬ mously to the correct interpretation of fertilization, for reasons which are clear from Van Beneden's figures (Fig. 4): In contrast to the human being (Fig. 3) or the sea urchin (36) the individual chromosomes, which in Ascaris megalocephala are only two in number per haploid set, can he seen clearly before, during, and after the actual fusion of the gamete pronuclei even with the relatively primitive cytological procedures in use in the 1880's. With Van Beneden's study it became evident (a) that the particular nuclear components which are contributed equally by both parents to the zygote are the chromosomes per se, and (î?) that perfect replicates of these chromo-
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