1885.] MISS B. LINDSAY ON THE AVIAN STERNUM. 691 



do those of HofFmann, who describes the conuection of sternum and 

 clavicle taking place " durch an Kuorpelzelleu 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 corre- 

 spond with the earliest stages which I have dissected, figured in 

 Plate XLV. 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 cnmelus), (ii.) the 

 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 dissectedj 

 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 tberefore 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 by 

 the histological character of different parts ; this is a consequence of 

 the fact that the first change the difi"erentiating 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 tlie 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 Eeptilian 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 a priori that in the embryo traces of the Reptilian 

 interclavicle also might be present, either free or, as suggested by 

 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. 



