2 THE VOYAGE OF H.M.S CHALLENGER. 



this present paper will treat of that region only ; the limbs, limb girdles, and spine will 

 take up less space : these will be worked out for me by young fellow-labourers/ 



Bibliography. 



BojANUs, L., Anatome Testudiuis Europese. Vilna and Leipzig, 1819-1821. 



CuviER, G., Ossemens Fossiles, atlas, vol. ii., plates coxxxix.-ocxlvi., 1836. 



Eathke, H., Uebei' die Entwickelimg der Schildkroten. Brunswick, 1848, 



Agassiz, L., tlontribiitions to the Natural History of the United States, vol. ii., jjart .3 ; Embryology of the 



Turtle. Boston, 1857. 

 Owen, R, Osteological Catalogue of the Museum of the Eoyal College of Surgeons England, vol. i. 



pp. 168-214, 1853. 

 Gray, J. E., Catalogue of the Shield Eeptiles in the British JMuseum. London, 1855 ; and also a series 



of Papers on the Testudinata in the Proceedings of the Zoological Societj', 1861-1873. 

 Huxley, T. H., Croonian Lecture, Proceedings of the Eoyal Society, November 18, 1858, p. 18. 

 Elements of Comparative Anatomy, pp. 219-244. London, 1864. 

 Anatomy of the Vertebrated Animals, pp. 197-208. London, 1871. 

 Parker, W. K., Shoulder Girdle and Sternum, Eay Society, pi. xii., pp. 133-141. London, 1868. 

 GuNTHEE, A., Eeptiles of British India, Eay Society, 1864, plates i.-vi., pp. 1-55. 



On Gigantic Land Tortoises, Philosophical Transactions, 1875, part 1, plates xxxiii.-xlv., 



pp. 251-284. 

 On the Gigantic Land Tortoises, Living and Extinct, in the Collection of the British 

 Museum. London, 1877. 

 Balfour, E. M., On the Development of the Elasmobranch Eishes. London : Macmillan & Co., 1878. 

 Marshall, A. Milnbs, On the Development of the Cranial Nerves in Birds, Quarterly Journal of 

 Microscopical Science, vol. xviii., new series, plates ii., iii., pp. 1-31. 

 Note on the Development of the Olfactory Nerve and Olfactory Organ of Vertebrates, 

 Proceedings of the Eoyal Society, February 13, 1879, p. 324. 



Fi7'st Stage. Emhryo, 3^ lines long. — The body is composed of about twenty-seven 

 somatomes, and the ventral surface of the body from the pericardium to the commence- 

 ment of the caudal region (composed of four or five somatomes) is wide open. 



There are evident thickenings where the limbs will be formed. The ventricle (PL I. 

 fig. \,h) is at present a large looped vessel, the auricles being rudimentary. 



The mesocephalic flexure is perfect, and the hind-brain (C 3) is very long, showing- 

 ten or twelve thickenings on each side below, from which the nerves are arising. The 

 mid-brain (C 2) is large, and completely bent upon itself, and the fore-brain (C 1), which 

 is inferior in position, is separated from the mid-brain, by a considerable pineal enlarge- 

 ment. The pituitary body has not begun to be formed. 



The nasal folds (ol.) are already formed, and the eyeball is seen as an imperfect ring 

 enclosing the large lens ; each of the auditory sacs (aii.) is a short-necked lagena with 

 an oval mouth, and having a very definite rim. 



Four clefts {cl. 1-4) are apparent, lessening in size from before backwards, the open- 

 ing of the last not being clear ; they are surrounded by a clear line of cells, possibly 

 derived from the hypoblast which lines the slits. 



' The sections of the head were all prepared for me by my son, Mr W. N. Parker. 



