Slide showing two diagrams of the 'Trochophores of Patella at different stages.' Slide shows two diagrams of the 'Trochophores of Patella at different stages.' Diagrams are labelled 'A' and 'B'. Title below diagrams reads 'Fig. 573, - A and B, Trochophores of Patella at different stages. In A are to be seen the circular blastopore and the two-foot elevations; in B the blastopore is drawn out, at the sides of it are the two mesoderm bands. (From Korschelt and Heider, after Patten.' Late 19th to early 20th century