Slide showing four diagrams regarding cleavage stages in snails. Slide shows four diagrams regarding cleavage stages in snails. Diagrams are labelled 'a', 'a'', 'b', and 'b''. The explanatory text below the diagrams reads: 'Fig. 92. - Above, the first cleavage stage in a snail, showing position of spindles for next cleavage division in left-handed (sinistral) cleavage (a) and right-handed (dextral) cleavage (a'). Below, same after cleavage; b, sinistral type, b', dextral type. (After Morgan.)'. Late 19th to early 20th century