Chapter 6: The Best-Laid (Body) Plans

1.) Early embryonic experiments in the 1800s led to the discovery of three germ layers. List their names and the organs that form from each.

Ectoderm – Epidermis of skin and its derivatives (including sweat glands, hair follicles), Epithelial lining of mouth and anus, Cornea and lens of eye, Nervous system, Sensory receptors in epidermis, Adrenal medulla, Tooth enamel,Epithelium of pineal and pituitary glands, Notochord, Skeletal system

Mesoderm – Muscular system, Muscular layer of stomach and intestine, Excretory system, Circulatory and lymphatic systems, Reproductive system (except germ cells), Dermis of skin, Lining of body cavity, Adrenal cortex

Endoderm – Epithelial lining of digestive tract, Epithelial lining of respiratory system, Lining of urethra, urinary bladder, and reproductive system, Liver, Pancreas, Thymus, Thyroid and parathyroid glands

2.) Describe the blastocyst stage in embryonic development.

The fertilized egg or zygote continues to divide. This leads to the creation of an inner group of cells with an outer shell. This stage is referred to as a blastocyst. The inner group of cells develop into an embryo while the outer group of cells turns into the membranes that support and shield it. The blastocyst reaches the uterus (womb) around day 5, and implants into the uterine wall on about day 6. The mother’s menstrual cycle causes the lining of the uterus to grow and prepare to support a human fetus and ultimately a baby. For the duration of the pregnancy the blastocyst sticks tightly to the uterus where it receives nourishment via the mother’s bloodstream.

3.) What is meant by “ontogeny recapitulates phylogeny?”

The study of the development of an embryo shows the repetition of an evolutionary stage during development and growth of a taxonomic group of organisms. So if somebody were to study embryos, a trend could be seen that the further along you go in the phylogenetic tree, the more stages of development the organism would pass through. This hypothesis was proven to be false. There are many examples of organisms who do not resemble previous evolutionary ancestors on their way to developing into an adult.

5.     What type of gene is Noggin and what is its function in bodies? Is Noggin an activator or a suppressor? (P111)Noggin is a type of HOX gene that helps with the body plan (such as the axes) but it is also involved with other organs. It is associated with another gene called BMP-4 when determining the top-bottom axis, where BMP-4 is the bottom gene (ventral side) and Noggin is the top gene (dorsal side). Whenever Noggin is active, BMP-4 is suppressed. Therefore, Noggin is a suppressor.

6.   Sea anemones have radial symmetry while humans have bilateral symmetry but they still have “similar” body plans – explain…The body-building genes of sea anemones and of humans and more complex organisms are the same. Sea anemones and humans have similar major body plan genes along mammalian head-to-anus axis to the oral-aboral axis of anemones. Sea anemones have similar belly-to-back genes to humans, but instead of along a belly-to-back axis, it was along a hidden axis called a directive axis that differentiates the sides of the organism into a left and right side. Although the end product of humans and sea anemones may appear quite different, both are based on the same basic recipe, and a long time of modification accounts for the visible difference between the body plans of a human versus a sea anemone.

Leave a comment


  1. It’s interesting to see that even organisms that seem extremely primitive have similar body plans to us humans.

  2. The statement “ontogeny recapitulates phylogeny” reminds me of the relationship between two types of organisms that are apparently totally unrelated, but yet are very close according to phylogeny: echinoderms and chordates. However, the relationship between the two is seen more clearly when one examines the larval stage of the echinoderm, which exhibits bilateral symmetry and cephalization, similarly to chordates. I am also amused by the names of some of the genes discussed in this book. As a researcher, does one get to name the genes one discovers essentially with impunity? I feel as though that might be one of the greatest perks of the job.

    • Right on with the ontogeny recapitulates phylogeny. Although there have been tweaks to the at idea with the new evidence from evo-devo. And yes, you get to name your gene, same as star discoverers get to name the star. But like most of science nomenclature there is first a standard convention, then there is the common name. See ML


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