In addition, while in the pronephron only vacuolization was noted, the structure of the liver had an atypical appearance and there was a considerable number of erythrocytes scattered among the hepatic chords which themselves were noticeably altered.
Source: wiktionary
The whole problem turned around a detailed understanding of the so-called Wolffian body, what modern textbooks in embryology designate as the pronephron and its associated Wolffian or pronephric ducts.
Source: wiktionary
Regression of the first clusters occurs before the total number of pronephron structures is established. The pronephrons induce the development of the mesonephrons. As the pronephrons regress, the mesonephrons appear.
Source: wiktionary