In this alpha release of Hedera Services 0.26, we are excited to deploy previewnet support for HIP-410 (Wrapping Ethereum Transaction Bytes in a Hedera Transaction). and HIP-415 (Introduction Of Blocks).
HIP-410 adds a HAPI EthereumTransaction by which an account that was auto-created with an ECDSA(secp256k1) key can submit Ethereum transactions to Hedera by signing with its ECDSA key. (Standard Ethereum restrictions on the sender's nonce apply.) Please see HIP-410 for details, including a summary of some very compelling use cases that the EthereumTransaction enables---for example, "I want to use MetaMask to create a transaction to transfer HBAR to another account".
HIP-415 also anticipates such use cases by standardizing the concept of a Hedera "block"; this is important for a full implementation of the Ethereum JSON-RPC API. The definition is simple: One block is all the transactions in a record stream file. The block hash is the 32-byte prefix of the transaction running hash at the end of the file. And the block number is the index of the record file in the full stream history, where the first file had index 0.