ON THE DEVELOPMENT OF THE HEART IN MARSUPIALS. 459 



35. The Early Development of the Heart and Anterior 

 Vessels in Marsupials, with Special Reference to 

 Perameles. By Katharine M. Parker, B.Sc. (Lond.), 

 Assistant in the Department of Zoology, University 

 of London, University College *. 



[Received May 25, 1915 ; Read June 8, 1915.] 

 (Plates I., Il.t and Text-figm^es 1-25.) 



Index. Page 



Introduction 459 



Description of Stages 460 



Summary and Discussion 486 



(a) Development of the Heart 486 



(6) Development of the Cardinal Veins 493 



Pinal Summary and Conclusions 497 



References to Literature 497 



Introduction. 



The following piece of work was undertaken at the suggestion 

 of Professor J. P, Hill, for whose invaluable help in carrying it 

 out I am deeply grateful. The embryos studied are all from his 

 collection and text-figs. 1 and 6 were made from photographs 

 taken by him. 



I wish also to thank Miss E. A. Steele for her beautiful 

 figures of the model (Stage Y.) and Mr. F. 0. Pittock for much 

 help in making the model. 



The material aifords an excellent opportunity for the study of 

 the mode of development of the pleuro-pericardial canals, the 

 origin and clifFerentiation of the endothelial heart-tubes, and the 

 method of fusion of the lateral primordia of the heart, and it 

 was in the hope of making some progress towards the solution of 

 the interesting problems of early cardiac development that the 

 work was undertaken. 



The general arrangement of the pericardium, heart, and aorta 

 in the early stages (viz. Stages I. to IV. inclusive) can be 

 accurately determined by graphic reconstruction, Figui'es 1 to 5 

 (PI. I.) were all obtained by this method, and are intended to 

 give some idea of the relations of the pericardium and heart to 

 the gut and the brain-plate in successive stages. 



In the next stage (Stage V.) the curvature of the heart makes 

 it impossible to represent the i-elations of the parts accurately in 

 two dimensions. A wax-plate reconstruction was therefore made 

 (see PI. II.), and as all the vessels of the head which were 



* Communicated by Prof. J. P. Hill, D.Sc, F.R.S., F.Z.S. 

 t For explanation of the Plates see p. 499. 



Prog. Zool. Soc— 1915, No. XXXII. 32 



