Embryology - 18 Jun 2024 Expand to Translate
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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

The studies of C. H. Heuser were supported in part by a research grant, RC-4212, from the National institutes of Health, U. S. Public Health Service.

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

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