. Text-book of embryology. Embryology. GASTKULATION 39. As will be gathered from an inspection of Fig. 24 the gradual covering in of the yolk-cells takes place in an eccentric fashion. On the side opposite to that on which the original involution groove makes its appearance there is comparatively little displacement of the boundary between large cells and small, while on the side where the groove is the displacement is relatively great—from a to d in the diagram. Intermediate points of the boundary between large and small cells are displaced more or less according to their greater or less prox
. Text-book of embryology. Embryology. GASTKULATION 39. As will be gathered from an inspection of Fig. 24 the gradual covering in of the yolk-cells takes place in an eccentric fashion. On the side opposite to that on which the original involution groove makes its appearance there is comparatively little displacement of the boundary between large cells and small, while on the side where the groove is the displacement is relatively great—from a to d in the diagram. Intermediate points of the boundary between large and small cells are displaced more or less according to their greater or less proximity to the point of original involution. As regards the method by which the yolk-cells become covered in, it would appear that the " dorsal" lip of the groove advances over the yolk by 9- a process of overgrowth, while at those parts of the boundary where there is no invagination-groove the process is one of delamination. The growth of the dorsal lip is clearly indicated by the outline of the yolk-plug in sagittal sections which indicates distortion by pressure from the dorsal lip. It will be realized from what has already been said that the outer lip of the circular groove (Fig. 24, d) is simply the rim of the gastrula-mouth or proto- stoma and that the preceding stages are above all characterized by this rim being incomplete. In other words the activity concerned in the involution of the gas- trular rim is accentuated at one point (a) while it is suppressed to such an extent elsewhere as only to become apparent at a comparatively late stage when the edge of the small- celled region has already spread to a great extent over the yolk-cells by a process of delamination. It will also be realized that it is not strictly accurate to speak of the circular area bounded by small cells as representing the gastrula- mouth until it is completely enclosed by the gastrular rim. The internal changes which accompany the phenomena just described are illustrated by the sagittal s
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