What are the 35 thoughtrons of the causal body?
In In Chapter 43 of Autobiography of a Yogi, Paramhansa Yogananda relates what Sri Yukteswar told him when the latter appeared to him after his own mahasamadhi:
Man as an individualized soul is essentially causal-bodied. That body is a matrix of the thirty-five ideas required by God as the basic or causal thought forces from which He later formed the subtle astral body of nineteen elements and the gross physical body of sixteen elements.
Here is a link to that chapter, where you can read more about those 35 elements. I hope it helps you in some way.