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Show 1885.] MISS B. LINDSAY ON THE AVIAN STERNUM. 691 do those of Hoffmann, who describes the connection of sternum and clavicle taking place " durch an Knorpelzellen reiches Bindegewebe," in sections of Carbo cormoranus " von zwei Tage alten Embryonen." Gotte's sections of 4-5 days' chicks probably correspond with those called chicks of 6 days' incubation in the Cambridge Morphological Laboratory. In these, however, I fail to detect the darker tissue which he describes as connecting sternum and clavicle. They correspond with the earliest stages which I have dissected, figured in Plate X L V . figs. 9, 10, 12, 14. The nomenclature used throughout this paper is explained by Fig. I. p. 690. Part II. DETAILS OF EMBRYONIC DEVELOPMENT IN FIVE TYPES, namely (i.) the African Ostrich (Struthio camelus), (ii.) Guillemot (Uria troile), (iii.) the Gull (Larus), (iv.) the Chick, and (v.) the Gannet (Sula alba). These embryos were not examined in sections, but were dissected, and this for two reasons : first, comparison of stages is easier if one method of preparation is employed throughout; and the younger specimens can be dissected, whereas the older ones, with which it is safest to begin, cannot be made into sections; secondly, sections are useless in studying the development of muscles, which is of necessity intimately connected with that of bones, and may therefore give some clue to their history. A chick in which the pericardial cavity is not yet closed can be dissected with perfect accuracy under a strong lens : indeed dissections can be made at a stage so early that they are useless, since the microscope shows little difference between various cells, and there is no means of checking the results of dissection bv the histological character of different parts ; this is a consequence of the fact that the first change the differentiating cells undergo is a change in their firmness and closeness. Plate XLIV. figs. 3-5 correspond with the stage called " early 6th day " in the Cambridge Morphological Laboratory, sections of which show comparatively little differentiation in the cells of the future shoulder-girdle. (i.) THE OSTRICH. (7 individuals, embryos respectively of 27, 25, 21, 15, 10, 7, and 4 days' incubation.) The adult Ostrich still presents certain Reptilian features: to wit, (a) the presence of two claws on the 1st and 2nd digits of the wing, while other birds that possess such claws, for example Rhea and the Swan, have only one, and most birds have none; (b) the broad coracoid, consisting of two parts separated by a foramen. These facts appeared to render it possible dpriori that in the embryo traces of the Reptilian interclavicle also might be present, either free or, as suggested bv Gotte's theory, in the form of a rudimentary keel; for the absence of the interclavicle is not implied in the loss of clavicles, as may be seen from its existence in the Crocodile ; but, on investigation |