|Description||Slide shows four diagrammatic drawings of 'localisation of organ-forming substances in Ascidians'.|
Diagrams are labelled 'A', 'B', 'C', and 'D'. Each diagram has other labels which are explained in the following key: 'Fig 56 - Localisation of organ-forming substances in Ascidians. Views of eggs of Styela. Yellow cytoplasm containing mitochondria (yp) small circles. Yolk (Gy) stippled. Clear cytoplasm (cp) white. A, before fertilisation showing germinal vesicle (GV), yellow cytoplasm evenly distributed over the surface. B, immediately after fertilisation, showing clear region (kp) derived from germinal vesicle at animal pole, yellow cytoplasm streaming down to vegatative pole. C, the yellow cytoplasm forms a cap at the vegatative pole (yz) containing the male pronucleus. The clear cytoplasm forms a layer just above it. D, Left side view of egg just before cleavage, showing yellow crescent (yc) and clear crescent (cp) posterior, and grey crescent (Gc) anterior. (From Conklin, Chap. IX of Cowdry, General Cytology, Chicago, 1924).'