Observations on the Development of Ornithorhy nchus. 313 



We desire to express our indebtedness to Professor Anderson Stuart, in 

 whose laboratory most of our work has been done, and to Dr. Gr. H. F. 

 Nuttall, whose book has been of great service in giving an account of the 

 work of other observers whose original communications we have been unable 

 to consult. 



Observations on the Development of Ornithorhy nchus. 



By J. T. Wilson, M.B., Professor of Anatomy, and J. P. Hill, D.Sc, Lecturer 

 on Embryology, University of Sydney, N.S.W. 



(Communicated by Sir William Turner, K.C.B., F.B.S. Beceived December 5, — 



Bead December 14, 1905.) 



(Abstract.) 



The paper treats of certain stages in the intrauterine development of the 

 egg of Ornithorhynchus. 



The stages dealt witb include the following : — 



(1) An early (eight-celled) stage of segmentation. 



(2) A stage manifesting a later phase of the segmentation-process. 



(3) A stage of early germinal-layer formation, in which the cellular blasto- 

 derm is almost exclusively arranged in the form of a much-attenuated 

 epithelial membrane covering part of the yolk-mass. The arrangement on 

 the whole resembles that illustrated in fig. 22, Taf. 8, and fig. 33, Taf. 9, in 

 B. Semon's work on " Monotreme Development."* Certain differences are, 

 however, noticeable, more especially as regards the absence of the deeply 

 placed nuclei figured by Semon. 



(4) A stage of more advanced germinal-layer formation, characterised by 

 the complete differentiation of the yolk-entoderm and of a lenticular mass of 

 cells connected with the outer layer of the vesicle, which would seem to 

 represent an early condition of " primitive knot " formation. This stage 

 represents that of the completed " first phase of gastrulation." 



(5) A stage exhibiting the characteristics of the " second phase of gastru- 

 lation." These include the presence of a fully-developed " primitive knot " 

 which may henceforth be designated as " archenteric," owing to the formation 

 in connection with it, by invagination, of an " archenteric," or " gastrulation- 

 cavity." Meanwhile there has also developed — quite independently of, and 



* ' Zool. Forsclmugsreisen in Australien,' etc., 1894, Bd. 2, Lief. 1. 

 VOL. LXXVIII. — B. 2 B 



