The following is a further account of the embryology of Pieris rapæ dealing with development from the time of germ layer formation, Eastham, 1927, to the time that those germ layers have become differentiated into the several systems of organs to be found in the caterpillar. Embryological differentiation, i.e ., fate of germ layers, rather than histological differentiation is described. Cytological details have been largely ignored, and since germ cell differentiation is so largely a cytological matter this part of the story is treated superficially, with the intention of dealing with it as a special problem later. The interesting morphological subject of head development has been treated at some length on account of the special interest raised by three important recent accounts dealing with this subject—two anatomical, Snodgrass, 1928, and Denis, 1927, and the third embryological, by three collaborators, Leuzinger, Wiesmann and Lehmann, 1926.