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My aliens in my generation ship story are reptilian with some mammal characteristics.

They have 2 hands, each of which has 6 fingers. 2 of those fingers are thumbs and the thumbs are on opposite sides of the hand. This allows them to get a better grip.

Their feet are shaped just like ours but with 6 toes instead of 5.

They have nostrils but no real nose. Their eyes are like cat's eyes. They have 3 antennae at the top of their head for hearing.

Their skin is scaly. I mean sure, human skin close up with the naked eye looks scaly and on a microscope looks like actual scales but my aliens truly do have scales and not just overlapping skin flakes.

Their pregnancy I think is the weirdest of all their weirdness. It starts off like a human pregnancy but with multiple eggs ovulating(10-15 is average). Then the eggs temporarily implant in the womb as the shell develops. Once the shells are fully formed(unlike snake eggs, they don't fuse(the shell is too hard for that)) the alien goes into labor. But this isn't a live birth, it is just the first stage of the pregnancy finishing and second stage starting. After the alien lays eggs, she puts them in her pouch to keep them safe and warm. After a few months of incubation, the eggs hatch and the mother produces milk.

Now I first thought that the chest would be a good place to have the pouch but then my momma told me that maybe I should have it on the abdomen kind of like a kangaroo and that if it was on the chest, these human shaped aliens would lose their balance and possibly break their eggs.

So here are the creatures I used to make these aliens:

Arthropod

Arthropod in general for the antennae

Cat

Cat for the eyes

Lizard

Lizard in general for the nostrils without a nose and the scales

Human

Human for the shape and locomotion

Kangaroo

Kangaroo for the pouch

But I am not sure if I should have the pouch on the chest or on the abdomen. On the one hand, a pouch on the abdomen would be safer. On the other hand a pouch on the chest makes more sense to me in terms of distance to milk supply. I mean where else would you expect milk supply on a humanoid creature than the chest?

So where should the pouch go, on the chest closer to the milk supply but at higher risk of breaking eggs or on the abdomen further from the milk supply but safer for the eggs?

I am not sure how to classify these creatures. I mean there is implantation like in placental mammals(except without a placenta and only temporarily as the shells develop). But then there is egg laying like monotremes. And a pouch to incubate the eggs like a marsupial. And scaly skin like reptiles. And milk production like all mammals

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    $\begingroup$ "I mean where else would you expect milk supply on a humanoid creature than the chest?" Note that this is humanoid, not human. Evolving aliens won't necessarily look exactly the same as humans, so there's no reason why milk wouldn't be produced within the pouch like a kangaroo $\endgroup$ – Zxyrra Nov 16 '16 at 5:15
  • $\begingroup$ @Zxyrra Good point about lactation teats in the pouch, & like a knagaroo. $\endgroup$ – a4android Nov 16 '16 at 7:21
  • $\begingroup$ They're your custom species; do with them whatever suits the needs of your story. $\endgroup$ – Frostfyre Nov 16 '16 at 13:13
  • $\begingroup$ As others stated the abdomen makes more sense, BUT for another reason: with upright creatures, the upper body moves far more (bending over, rotatinc etc) than the lower body, since you want these eggs to develop, less movement is better $\endgroup$ – Hobbamok Mar 13 at 13:22
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The abdomen makes more sense in general. Lower center of gravity makes it easier to balance.

Most animals produce milk from their lower body. The exceptions are apes, which can hold their babies in their arms, so it makes sense to feed them from there too. If the baby is being held by the pouch, though, milk should be produced from inside the pouch, which is where kangaroos make it.

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You might want to study the life cycle of the platypus and echidna (monotremes), which lay eggs and then feed milk to the hatchlings in a pouch, exactly as your reptiles do.

Implantation

Minor science note - if a fertilised cell is going to be inside an egg, it will NOT implant.

Implanting is something that only placental mammals do. If the zygote implanted in a womb wall, you couldn't form a complete shell around it.

There wouldn't be a womb at all, because only placental mammals need a womb. There would be an egg tract, like birds and reptiles (and monotremes) have, which would be continuous, not bulging out into a womb at any point. By the time the fertilised cell has travelled the entire length, it has the yolk, white and shell around it, and the egg is ready to be laid.

Location of pouch

Abdomen, absolutely. The chest is hard, to protect the heart and lungs.

Unless you were making so many eggs you could afford to lose batches of them every time you got in a fight, evolution would favour tucking the eggs as deep into the body as possible.

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