As my tentacles pick up the girl and bring her in to the breeding sac her hands and feet are guided into special pouches in the interior of the sac, think of these pouches as mouths without teeth or tongues and that don't lead to a throat/stomach.
Once the girls limbs are placed inside the pouches they close holding her in place, the tentacles are still involved here, keeping her body still so that she doesn't injure herself or her new owner with any form of struggling. As soon as the pouches are closed their 'lips' shoot forwards along her arms and legs, at least as far as the joint, and begin to meld with her skin, removing the unnecessary tissue below the meld line. My blood vessels are connected to her circulatory system and from there her fate is permanent. My much stronger heart overpowers hers and starts to circulate my blood through her veins carrying oxygen and nutrients through her system (let's just handwave the problems that would be caused by blood antibodies, type, etc.). Her body is now completely dependent on mine for its survival, her heart lungs and digestive system start to atrophy as they are no longer needed - by the time she births my first child she'll be long past the point where it would be even theoretically possible to remove her from my body.
From here on out it's no longer accurate to describe her as a separate entity, she still thinks independently but she is now a component of my body. She would be understandably at least a little worried by now, and as we share the same bloodstream that would be a problem (her flooding my system with stress chemicals is not a good outcome). Two tentacles move carefully towards her mouth and anus, while they produce their own lubrication they are still rather large and are inserted slowly. Once they are sufficiently deep in her arse and throat they meld (as with the section that merged her arms and legs with my tissue) at the point of entry and begin pumping a thick, sweet, warm mixture of calming chemicals and fertility drugs into her from both ends. This will be a constant factor of her new life as part of me. As this takes effect she relaxes and the sac closes, before it starts to fill with a pH neutral, antibiotic, warming, gel covering her completely as it keeps her safe and completely immobile. She's now ready for her first insemination.
As the insemination tentacle starts slowly stroking her pussy and clit the tentacles in her mouth and anus begin slowly thrusting in and out, mimicking her getting spitroasted by two very well endowed men - she'll love this before too long, it's going to last for the rest of her life. The insemination tentacle slowly pushes its way up inside her virgin pussy and continues up past her cervix, entering her womb directly, and pumping about a litre or two of my semen equivalent - filling and distending her womb, stretching it out to accommodate the growth of my young. Her eggs are useful in this process as an initial nutrient supply (her own DNA will be removed from the cell by my gametes before cell division occurs) and after insemination gestation will take about 10 weeks. My child will be born as a large spherical mass of flesh that will grow and survive under its own direction. The chemical changes made to her blood, as well as the large quantities of fertility supplements I'm pumping in at both ends, will give her hyper-exaggerated lactation, two sucker tentacles attach themselves to her nipples and milk her constantly as her breasts grow to ridiculous proportions.
Obviously so rapid a pregnancy (combined with the changes made to her body and the fact that she will have no rest between breedings) would have a harmful effect on her body and health. She should survive for at least 10 pregnancies - with some variance around that figure depending on the girl. Once she dies her body can be absorbed as nutrients and a new girl will be found.
Anybody enjoy that? I'm kind of bored at the moment and would appreciate a few writing prompts/requests.