Foundations Practical - Week 1 to 8

From Embryology

Foundations Practical: Introduction | Week 1 and 2 | Week 3 and 4 | Week 1 to 8 | Week 9 to 36 | Neonatal | Critical Periods | Additional Resources | Quiz

The Embryonic Period

Development of Organs and Tissues

You have now been through the earliest stages of embryonic development. From the original simple trilaminar embryo all the organs and tissues will now begin to develop, details of these processes require more time than we currently have available. What we will now look at are the changes in the external embryo appearance during this period.

Historically the embryonic period was also divided into 23 stages (I do not expect students to identify embryo stages).

Human Carnegie stage 1-23.jpg

Week: 1 2 3 4 5 6 7 8
Carnegie stage: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Key features to note are:

  • changes in overall size (CRL)
  • surface bulges that reflect organ development
  • development of the upper and lower limbs
  • development of the head

This page is about getting an overview of the external changes that occur during the embryonic period. In order to observe and understand the internal changes occuring in organ development is beyond the scope of this current practical.

Changes in Overall Size

In the first 8 weeks the embryo grows from 0.1 mm to about 3 cm in length.

Actual Size 5cm ruler.jpg

Enlarged Size Stage1 size with ruler.jpg

During this time the embryo size is also measured from "crown" to "rump" or the Crown Rump Length (CRL).

Stage13 crown rump length.jpg

Surface Bulges Reflect Organ Development

Stage13 surface bulges.jpg

Surface of the Stage 13 Embryo showing surface features reflecting surface and internal development. Carnegie stage 13

Week 8 - End of Embryonic Period
Stage23 bf3.jpg Stage23 bf4.jpg Stage23 bf5.jpg Stage23 bf6.jpg
Stage23 MRI 3D01 icon.jpg
 ‎‎Embryo Surface
Page | Play
Stage23 MRI 3D02 icon.jpg
 ‎‎Embryo CNS
Page | Play


During this period the parts of the conceptus not contributing to the embryo will contribute the fetal component of the placenta and the embryonic/fetal membranes.

Gray0032.jpg Placental membranes.jpg

This topic will also be covered in detail in BGD.

Timeline Overview

Human development timeline graph 01.jpg

Human development timeline graph icon.jpg
 ‎‎Human Timeline
Page | Play

Additional Information

Additional Information - Content shown under this heading is not part of the material covered in this class. It is provided for those students who would like to know about some concepts or current research in topics related to the current class page.

Take the Quiz

Open the table below, select your answers, click submit, then reopen the table to see your result.

Quiz - Week 1 to 8  


1 Which of the following terms best describes development during the first 8 weeks after fertilisation.

Blastula period
Gastrula period
Conceptus period
Embryonic period
Fetal period

2 Growth in crown-rump length (CRL) is greatest during the first 2 weeks of development.


3 By the end of embryonic development the entire body and organs have formed and just growth of these structures occurs during the fetal period.


4 Which of the following statements is correct about congenital abnormalities in week 1-8:

congenital abnormalities that occur during this period will be corrected later in development.
congenital abnormalities that occur during this period can be more serious than those later in development.
congenital abnormalities generally do not occur this early in development.
congenital abnormalities occur only from genetic causes during this period.
congenital abnormalities occur only from infections during this period.


Note - linked terms in the list below are external resources to the current class content.

  • Bulge - A non-technical term describing a protruding part or an outward curve or swelling.
  • CRL - Acronym for Crown Rump Length, a staging measurement of embryo and fetal size.
  • Crown Rump Length - (CRL) A staging measurement of embryo and fetal size. Measures length from the top curvature (crown) to bottom curvature (rump) in mm. (More? Crown Rump Length)
  • embryonic period - Term describing human development up to week 8 (GA week 10).
  • Gestational Age - GA Clinical age from first day of the last menstrual period (LMP) about 2 weeks before fertilization occurs. Gestational Age, not Fertilization Age is generally used clinically.
  • MRI - magnetic resonance imaging (nuclear magnetic resonance imaging, NMRI) An imaging technique that uses a powerful magnetic field to align the nuclear magnetization of hydrogen atoms in water in the body.
  • Organ - A collection of tissues joined in a structural unit to serve a common function.
  • Stage - Carnegie stage, a historic staging system for human embryonic period from fertilization, between week 1 to 8 (GA 3-10) of development. (More? Carnegie Stages)
  • Tissue - A group of cells that perform a similar function.
  • ultrasound - A non-invasive technique using sound waves in the ultrasonic (above our hearing range) frequency for visualizing and prenatal diagnosis of several features of development of: follicles in the ovaries, the gestational sac, fetus in the uterus, fetal parameters, and the placenta. (More? ultrasound)


Foundations Practical: Introduction | Week 1 and 2 | Week 3 and 4 | Week 1 to 8 | Week 9 to 36 | Neonatal | Critical Periods | Additional Resources | Quiz

Glossary: A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Numbers

Cite this page: Hill, M.A. (2024, April 13) Embryology Foundations Practical - Week 1 to 8. Retrieved from

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G