porifera

 the ovarian and uterine
​systems are related

                                     pathology

move from
external fertilization
to
 internal fertilization

           the placenta!

The syncytium is the outer cell layer of the placenta that is in direct contact with maternal blood. 

The seahorse is an ovoviviparous fish.
It exhibits male pregnancy...
The female seahorse deposits eggs into the male seahorse's unique brood pouch where the eggs are fertilized
and spend 24 days developing.

   cnidaria

​The function of the placenta is to supply the fetus with oxygen and nutrients and remove wastes. It exhibits a large exchange surface area composed of a thin membrane separating maternal and fetal circulations.  

echinoderms

      the egg!

       internal fertilization

      oviparous, viviparous, ovoviviparous

  external vs internal fertilization

  fish - amphibians - reptiles - aves - mammals

ovulation - fertilization - implantation

Amniotic eggs evolved over 300 million years ago (small lizards evolved from the amphibious reptiliomorphs Carboniferous geologic period) separating amphibians from reptiles, birds and mammals. Amphibians are restricted to wet environments to keep their eggs moist but the amniotic egg permitted animals to expand on land. Birds evolved a hard calcium carbonate shell on their egg and mammals eliminated the shelled egg in favor of an internal system exhibiting similar embryonic ​layers within the mother's uterus.
The advantages of the amniotic egg include:
 access to terrestrial environments
less competition for pools
decreased predation

   the amazing placenta!

asexual reproduction:
 1. mates aren't required
2.  fast process with many offspring produced in a short time period
 
  Sexual reproduction:
increase in variation occurring due to
a. crossing over
b. the fusion of gametes.

  chordates

       remember from 111...
crossing over occurs when
​ homologous chromosomes
​(chromosomes of the same pair)
are lined up and a part of the chromosomes are traded between the pair so that the two chromosomes still contain the same genes but may have different sequences in the genes.

 external fertilization:
* only occurs in the water (fish/amphibians)
​* female lays eggs/male sheds sperm over them to fertilize
* many more eggs released  than are fertilized or mature
efficiency?
*excess wasted sperm/egg
*more eaten or die due to environment than survive 

   annelids

 but first...fertilization

Losing an eggshell presents challenges.
The maternal and fetal cells must be kept separate to avoid "rejection". The separation
is established through a cell layer called the syncytiotrophoblast (formed with the help of syncytin) the outermost layer of the placenta pressed up against the uterus forming an impervious wall. 

​  the bottom line...

           all reproduction strategies serve to:
​1. produce a maximum number of surviving offspring
​2. ensure the species survives
3. expend the least amount of energy to do both 

arthropods

  vertebrate reproduction

platyhelminthes

Seahorse fathers to provide nutrients to their developing embryos in the form of
fats and calcium which help the embryos to build the bony plates under their skin.
The brood pouch also protects the embryos by secreting antibacterial molecules.
A week before they are released from their brood pouch, the young seahorses
break out of their membraneous coverings and swim about the inside the brood pouch stretching the pouch initiating the release of estrogen which initiates the release of the fry from the pouch. 

      but this is what we think of
        ​when we think chordate... 

an ancient retrovirus gene mutated, evolving into a gene that codes for syncytin. This protein permits the blastocyst to fuse into the uterine wall...
​some retrovirus infected an egg-laying vertebrate, settling in the animal’s egg cells. then that egg was fertilized. The protomammal that was hatched
had copies of that virus’ DNA in all its cells.

        fertilization (or not...)

  move to sexual reproduction from asexual reproduction

           endocrine control of oogenesis

the placenta permits
two mammals to coexist although being genetically very different since half of it is fetal and half maternal.

nematodes