My intention for some time past has been to follow up the Morphology of the Fish’s skull by that of the Mammal; and as amongst the “Placentalia” the Guineapig ( Cavia aperea ) takes a very low place, it was chosen as the type to work out. I have been led to change my plan, however, and to take a medium type by an unexpected supply of materials kindly put into my hands, in November 1871, by my friend Mr. Charles Stewart; these were about seventy embryos of the Common Pig, a considerable number of which were barely two thirds of an inch in length, whilst others measured 6 inches in a straight line from the snout to the tuberosity of the ischium *. As the tissues in the earlier stages were only in a nascent condition, the greatest care has been taken to harden them for slicing into sections and for dissection from without inwards; and no labour has been spared in this matter the sections being made after the hardened embryos had been imbedded in solid paraffin. These extremely thin objects were coloured with an ammoniacal solution of carmine, and then transferred to slides, on which they were mounted in acid glycerine. The coarser sections of the larger embryos, to be used as opaque objects, were made without imbedding, after the specimens had been immersed in a dilute solution of nitric or muriatic acid, to which had been added some chromic acid; in the former way I have been able to obtain views of the tissues of the earliest stage under a magnifying-power of as much as 600 diameters, although about 50 diameters has been found to be the most useful, showing, as such a lesser enlargement does, the various parts in relation to each other, and enabling the eye to follow the granular thickenings which are becoming differentiated into special tissues.