January 21, 2015, Embryonic development of birds and reptiles (compared with mammals)

Mammal embryos cleave like sea urchins (because mammal eggs have little yolk) but gastrulate like birds (because mammals evolved from reptiles, with lots of yolk)

Reptiles evolved four extra-embryonic membranes:
And then birds and mammals inherited all four of them.

Chorion: Ectoderm & Mesoderm

    Surrounds all the rest (of the embryo and the other e.e. membranes
    Is in direct contact with the uterus (in mammals)
    [of course, mammals are the only vertebrates that have uteruses]

The space between chorion & amnion is the Extraembryonic Coelom

Amnion: Ectoderm & Mesoderm

The space enclosed inside the amnion is the Bag of Waters.

Yolk sac: Endoderm & Mesoderm
is an outpocketing of the middle of the digestive tract (Stores yolk for food)

Allantois: Endoderm & Mesoderm is an outpocketing of the hindmost part of the digestive tract (Stores urine) (very well-developed blood vessels)

Birds and mammals inherited all 4 of these extra-embryonic membranes.
("extra" in this sense means "outside", rather than "more than usual")

The umbilical cord connects all four of these to the body itself, and contains two arteries and one vein.
(Urine can sometimes squirt out of the belly button in young children)

Turtles, snakes, lizards sometimes hatch with a large yolk sac protruding from them. This will be physically retracted into the body in a few days; don't cut it off!

Many species of snakes incubate their eggs inside the uterus, instead of laying eggs.

Most reptiles have flexible "leathery" egg shells. Indentations in reptile egg shells often push on early embryos, which can result in two-headed offspring.

In mammal embryos, the body develops out of part of the inner cell mass. The large sphere of cells ("the trophoblast") develops into the chorion.

Mammal embryos digest a pathway into the wall of the uterus "endometrium" and crawl down into it until completely surrounded by uterine cells. (This digesting & burrowing is called "implantation")

digest through uterine tissues, including blood vessels, so that blood flows directly across the part of the chorion where the allantois is just inside.
Advantage: better permeability
Disadvantage: more bleeding & difficulty at the time of birth

Pigs, horses and many other kinds of mammals have much less of this tissue removal. Advantage: less damage to uterus at birth
Disadvantage: worse permeability; urine actually gets stored in allantois

Note: There is no kind of animal in which the circulatory system of the mother ever connects directly to the circulatory system of embryos (like a trailer hook-up).

Although that would seem to make sense; and some very weird methods of nutrient transfer have evolved. For example, live-bearing teleosts have placenta-like connections between the ovary of the mother and the perichondrium of the embryo, and in some species of sharks one of the developing embryos bites off pieces of other embryos and devours them. Julian Lombardi was the world's expert on these kinds of phenomena, wrote a whole large book on it, and for years was a member of this department and taught many courses.

He proposed a hypothesis that early live-bearing reptiles may have been the first to evolve the chorion, amnion etc. Previously, scientists had assumed (for no particular reason) that the reptile & bird type of egg and the Primitive Streak & Hensen's Node type of gastrulation must have first evolved in egg-laying primitive reptiles &/or advanced salamanders. If eggs fossilized better & more often, then we might know for sure.

Please try to invent experiments or other predictions that could settle this dispute conclusively. (Especially if the evidence will support Julian's theory.)

Possible exam questions:

*A) What are some testable predictions of the assumption that the reptile&bird pattern of development evolved first in egg-laying species? [I have just thought of one! Hint: I got the idea from Ichthyosaurs.]

B) Which would you expect to have the largest allantois? Humans or pigs?

*C) What do primates have in common with rodents that could explain the similarity of implantation of their embryos?

D) Which two extra-embryonic membranes are partly made of endoderm?

E) Which two extra-embryonic membranes are partly made of ectoderm?

F) Which extra-embryonic membranes are partly made of mesoderm?

G) Which extra-embryonic membranes are connected to the body by means of the umbilical cord?

H) Which two extra-embryonic membrane is directly in contact with the uterus?

I) The extra-embryonic coelom is the fluid-filled space between the amnion and the chorion?

J) When identical twins are both surrounded by the same amnion, then those twins must have become separated by which of the three alternative causes of identical twinning.

K) Conjoined twins both develop inside the same ___________ and ____________.

L) Conjoined twins each have a separate ___________ and ____________.

*M) Would it be possible for a species of reptile or bird to have identical twins?

N) Would it be possible for a species of reptile or bird to have conjoined twins?

O) If the true answer to one of the preceding questions were proven to be "yes", then what would be the answer to the other question? And explain why?

P) Why can't you observe belly-buttons on birds? Because feathers are not transparent.

Q) If an indentation in the flexible shell of a reptile pushed directly on the anterior end of the primitive streak, then what sort of birth defect might result.

R) Would you expect it to be possible for the same kind of birth defect to occur in a bird?

S) Is it possible for early mammal embryos to implant in the fallopian tubules or other tissues?

T) WHY is it possible for early mammal embryos to implant in the fallopian tubules or other tissues?

U) If a set of genetically identical triplets were born, with two of the three having been inside the same chorion and the third having been surrounded by a completely different chorion, then describe the events that must have happened to produce the bodies of the triplets.

V) If each of a pair of twins were implanted at different parts of a uterus, some distance apart, then what event must have occurred to produce this result?

W) If a pair of conjoined twins shared the same allantois, then what part of the body must they share?

X) What if a pair of conjoined twins shared the same yolk sac, what could you conclude?

Y) What if a pair of conjoined twins shared the same chorion, what could you conclude?

Z) What if a pair of conjoined twins shared the same amnion, then what could you conclude?


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