By monitoring the orientation of specimens in blocks and monitoring the orientation of sections and images taken from them, it was established that all (eukaryotic) flagella and cilia have the same inherent rotational asymmetry, and that there is a clockwise twist when looking along an axoneme from tip to base. The dynein arms point anti-clockwise.

The same principle could be extended to developing a dorso-ventral convention.

micro*graph and micro*scope contain images of Entosiphon where there is further evidence of asymmetries that would have been helpful in establishing dorsal and ventral.

Mike Sleigh's article