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Fig. 2. Examples of germ cell identification in non-model systems. (A) An adult
Mnemiopsis leidyi (a comb jelly of the phylum Ctenophora) in which
germ cells seem to arise by epigenesis. These hermaphrodites have eight rows
of gonads, each gonad containing an ovary and a testis behind each of the
eight rows of comb plates (also called ctene rows) (asterisks). (B) Close up
of area boxed in A. Germ cells are first identified in ctenophores after the
larvae hatch, next to the meridional canals that give rise to the ctene rows.
Multiple ovaries (black arrowheads) and testes (white arrowheads) develop on
either side of the canals. In this panel, eggs (asterisks) are being extruded
through the gonoducts. (C) Juvenile Blatta germanica cockroach
(phylum Arthropoda). Germ cells in these insects do not appear to be
determined by preformation. (D) The embryonic rudiment of B.
germanica forms on the surface of the yolk (yellow). (D, part i) Germ
cells (gz) are first identified at the posterior of the germ band, after
formation of the mesoderm (ms). (ii) As development proceeds, germ cells
continue to arise from the mesoderm of the coelomic sacs (c), which are being
formed in each segment in an anteroposterior progression. (iii) The number of
germ cells increases, and they populate the coelomic sacs of the segments from
which the gonad will form. c, coelomic sac; ek, ectoderm; gz, germ or
reproductive cells (genitalzellen); ms, mesoderm; st, stomodaeum
(Heymons, 1891 ). (E) A copepod
of the genus Cyclops (phylum Arthropoda). All copepods that have been
studied segregate germ cells by preformation. (F) Embryonic cleavages of
Cyclops fuscus are holoblastic and equal. (i) In the first cleavage,
dense granular material associates with only one of the centrosomes. (ii) The
resulting two-cell stage has the granular material in only one of the
blastomeres (orange). (iii) The granular material continues to be
asymmetrically segregated to a single blastomere (orange) in subsequent
cleavages. (iv) At the time of gastrulation, the cell containing the granular
cytoplasm has divided to give rise to two cells that are located at the tip of
the invaginating archenteron, which are the PGCs (Ug: Urgeschlechtszellen)
(Amma, 1911). (G) Late stage
embryo of the turtle Trachemys scripta (phylum Chordata), stained
with Alcian Blue for cartilage and Alizarin Red for bone. Reptiles seem to
segregate germ cells epigenetically. (H, part i) Section of an embryo of the
turtle Sternotherus odoratus at the three somite stage. Germ cells
are first identified at this stage of development, in two zones (Z) lateral to
the neural groove (NG). (ii) Close up of area boxed in i. PGCs in the germ
cell zone (Z) are distinguishable from somatic cells of the ventral ectoderm
(VE) as large cells with round nuclei and granular cytoplasm. AC, amnion and
chorion; DE, definitive endoderm; N, notochord; NG, neural groove; VE,
vitelline endoderm; Z, germ cell zone. Reproduced with permission from Risley
(Risley, 1933). Scale bars: 3
mm in A; 250 µm in B; 150 µm in C; 50 µm in E; 5 mm in G.
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