योनिमन्ये प्रपद्यन्ते शरीरत्वाय देहिनः । स्थाणुमन्येऽनुसंयन्ति यथाकर्म यथाश्रुतम् ॥ ७॥

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English Translation

Some jivas enter the womb to be embodied as organic beings and some go into non-organic matter-according to their work and according to their knowledge. 
 

Sri Shankaracharya

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English Commentary

Into wombs, combined with semen virile. Some ignorant fools go to take a body. The meaning is that the jîvas having a body enter the womb. Others, yet inferior after death, become immoveable, such as trees and the rest; ‘according to their karma, means according to karma performed by them, i.e., by the form of karma performed by them, in this birth. Similarly also , ‘ according to their knowledge,’ i.e., according to the nature of knowledge acquired by them. The meaning is that they take a body corresponding to them; for, another sruti says ‘they are born according to their knowledge.’

Sri Shankracharya