T. Thomson Flynn and the monotreme egg from oocyte maturation to germ layer formation

Abstract

Knowledge of oocyte development and the early differentiation of the germ layers in monotremes stems largely from two articles by J. P. Hill and T. Thomson Flynn. The completeness of their account was due to the large series of echidna ovaries and eggs collected on Tasmania by Flynn, an Australian biologist of whom a brief account is given. A striking finding in the oocyte and early embryo of monotremes was the presence of a latebra connected to the yolk bed beneath the germinal disc as described in birds and several reptiles. A further resemblance was the presence early in vitellogenesis of a striate zone beneath the zona pellucida. Cleavage resulted first in a lens-shaped blastodisc encircled by cells called vitellocytes that later fused to form a germ-ring. The blastodisc gave rise to a blastoderm that eventually became unilaminar but comprised two cell types identified as ectoderm and primitive endoderm. Eventually these segregated into two layers and formed a blastula. This sequence resembled that in marsupials except there was no obvious distinction between future embryonic and extra-embryonic regions. As the blastoderm extended over the surface of the yolk, it was preceded by the germ-ring that eventually played a role in forming the yolk navel. This was a unique feature of monotreme development. It is shown that Flynn played an important role in analysis of the material as well as in its collection.

Phylogeny and evolutionary history of the amniote egg

“…Downloaded 12.02.2021 from journal of anatomy, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1319107/pdf/janatphys00196-0163.pdf and https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1319085/pdf/janatphys00197-0060.pdf (copyright in public domain) Gatenby, 1922; Echidna aculeata: plate 1, figures 1-9 in Semon, 1894; Figure 14). These observations have been confirmed (e.g., Gatenby & Hill, 1924;Flynn & Hill, 1939;Hughes, 1993;Menkhorst et al, 2009;Carter, 2021 in this issue of the Journal of Morphology) and compared with the occurrence of a latebra in the yolk in sauropsids (Caldwell, 1887) and presence of white yolk and a latebra in Echidna (Semon, 1894). The distinction between white and yellow yolk goes back to Schwann (1847).…”

“…Gatenby (1922) reported details about the ripe egg of Ornithorhynchus paradoxus , including a description of the structuring of the yolk in an inner and an outer zone, and a latebra with a central core, a neck and enlarged upper area under the germinal disc ( O. paradoxus : plate 12, figures 1, 3 in Gatenby, 1922; Echidna aculeata : plate 1, figures 1–9 in Semon, 1894; Figure 14). These observations have been confirmed (e.g., Gatenby & Hill, 1924; Flynn & Hill, 1939; Hughes, 1993; Menkhorst et al, 2009; Carter, 2021 in this issue of the Journal of Morphology) and compared with the occurrence of a latebra in the yolk in sauropsids (Caldwell, 1887) and presence of white yolk and a latebra in Echidna (Semon, 1894). The distinction between white and yellow yolk goes back to Schwann (1847).…”

The shingled girl: Catherine Janet Hill and her contributions to embryology

“…The material then at her disposal was limited to the later stages of development in the platypus and echidna. In 1930, however, Theodore Thomson Flynn (1883–1968) had brought to London his large series of echidna ( Tachyglossus aculeatus ) specimens collected in Tasmania (Carter, 2021). As this material was processed, earlier stages became available, and Hill was able to revise and expand on her previous work (Hill, 1941).…”

“…Its future had preoccupied J. P. Hill for several years. It included an exceptional number of specimens from his early work in Australia that had been augmented over the years, not least by incorporation of the large series of echidna specimens collected by Flynn (Carter, 2021). There was primate material from many sources including unique specimens of gravid uteri of chimpanzee ( Pan troglodytes ) and lowland gorilla ( G. gorilla ) collected in The Cameroons by Fred Merfield (1889–1969) (Pijnenborg et al, 2011).…”

“…Catherine Janet Hill, known to family and colleagues as Katie, was the daughter of the renowned embryologist James Peter Hill FRS (1873–1954) (Watson, 1955). J. P. Hill is remembered for his studies on monotremes, marsupials and primates (Carter, 1999, 2021; Smith, 2020). Catherine Hill was a salaried research assistant in his laboratory at University College London (UCL) and made significant contributions to this work.…”