Paper - Developmental horizons in human embryos group X: Difference between revisions
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==Introduction== | ==Introduction== | ||
For convenience in classifying human embryos as to state of structural organization and age, the late George L. Streeter (i945, 1948, 1949, 1951) set up 23 age groups to indicate levels of development from the fertilised egg to embryos measuring 30 mm. in length, or approximately 47 days old. Beginning with stage xi, ovulation age 24 ± 1 days, Dr. Streeter carried the work forward through the remaining older groups. In preparing a survey of embryos of the first 3 weeks of development, it is convenient to begin with the oldest specimens of the period and thus consider the age group immediately preceding the one first completed by Dr. Streeter. | |||
==Age Group X, 4 to 12 Somites== | |||
===External Form, Size, and Age=== | |||
The most conspicuous event during age group x is the | |||
fusion of the neural folds. In the younger specimens the | |||
neural groove is open through its whole length. Fusion | |||
of the folds begins at about the stage of 7 somites, and | |||
by the end of the period the neural tube is closed from | |||
the otic region of the rhombencephalon to about the level | |||
of the latest-formed somite. Other important changes of | |||
form occur during stage x. The embryo progressively | |||
elongates, and the yolk sac continues to expand. At the | |||
same time the cranial portion of the neural folds grows | |||
forward and becomes distinctly elevated; the tail fold | |||
appears at the beginning of the period and overrides the | |||
yolk sac, and finally the whole embryo rises above the | |||
level of the yolk sac. The visceral components of the | |||
head begin to appear. Toward the end of the period, the | |||
increasing size of the heart makes the pericardial region | |||
a prominent feature of the external form. | |||
The task of compiling for the “Horizons” series a survey of this stage has been lightened by free use of the | |||
admirable study of human embryos, covering a slightly | |||
longer period of development (2 to 16 somites), that was | |||
published by Bartelmez and Evans in 1926. Their study, | |||
which was concerned chiefly with the early development | |||
of the nervous system, included all relevant embryos | |||
then in the Carnegie and the University of Chicago Collections, and incorporated references to specimens in | |||
other laboratories. Since it appeared, 3 additional embryos of stage x have been acquired by the Carnegie | |||
Collection; the University of Chicago specimens have | |||
been deposited there; and 5 embryos have been described | |||
from other laboratories. In all, counting both fully described specimens and those that have merely been | |||
cited incidentally, about 36 embryos having 4 to 12 pairs | |||
of somites are known to have been examined by the embryologists of the world. Of these, I3 are now in the | |||
Carnegie Collection (including 2 which are not good enough for use in the “Horizons”), and 6 more are rep- | |||
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Heuser CH. and Corner GW. Developmental horizons in human embryos. Description of age group X, 4 to 12 somites. (1957) Carnegie Instn Wash Publ 611, Contrib. Embryol., 36: 29-39.
Developmental Horizons in Human Embryos - Description of Age Group X (4 to 12 somites)
by Chester H. Heuser and George W. Corner
Chester H. Heuser | George W. Corner |
Introduction
For convenience in classifying human embryos as to state of structural organization and age, the late George L. Streeter (i945, 1948, 1949, 1951) set up 23 age groups to indicate levels of development from the fertilised egg to embryos measuring 30 mm. in length, or approximately 47 days old. Beginning with stage xi, ovulation age 24 ± 1 days, Dr. Streeter carried the work forward through the remaining older groups. In preparing a survey of embryos of the first 3 weeks of development, it is convenient to begin with the oldest specimens of the period and thus consider the age group immediately preceding the one first completed by Dr. Streeter.
Age Group X, 4 to 12 Somites
External Form, Size, and Age
The most conspicuous event during age group x is the fusion of the neural folds. In the younger specimens the neural groove is open through its whole length. Fusion of the folds begins at about the stage of 7 somites, and by the end of the period the neural tube is closed from the otic region of the rhombencephalon to about the level of the latest-formed somite. Other important changes of form occur during stage x. The embryo progressively elongates, and the yolk sac continues to expand. At the same time the cranial portion of the neural folds grows forward and becomes distinctly elevated; the tail fold appears at the beginning of the period and overrides the yolk sac, and finally the whole embryo rises above the level of the yolk sac. The visceral components of the head begin to appear. Toward the end of the period, the increasing size of the heart makes the pericardial region a prominent feature of the external form.
The task of compiling for the “Horizons” series a survey of this stage has been lightened by free use of the
admirable study of human embryos, covering a slightly
longer period of development (2 to 16 somites), that was
published by Bartelmez and Evans in 1926. Their study,
which was concerned chiefly with the early development
of the nervous system, included all relevant embryos
then in the Carnegie and the University of Chicago Collections, and incorporated references to specimens in
other laboratories. Since it appeared, 3 additional embryos of stage x have been acquired by the Carnegie
Collection; the University of Chicago specimens have
been deposited there; and 5 embryos have been described
from other laboratories. In all, counting both fully described specimens and those that have merely been
cited incidentally, about 36 embryos having 4 to 12 pairs
of somites are known to have been examined by the embryologists of the world. Of these, I3 are now in the
Carnegie Collection (including 2 which are not good enough for use in the “Horizons”), and 6 more are rep-
Cite this page: Hill, M.A. (2024, June 18) Embryology Paper - Developmental horizons in human embryos group X. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Developmental_horizons_in_human_embryos_group_X
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G