476 MISS K. M. PARKER ON THE 



dorsal to the heart-tube. The main trunk of each ends blindly 

 anteriorly and posteriorly and is of considerable size, causing a 

 bulging of the mesoderm of the somatopleure, which is thus 

 brought into contact with the mesoderm surrounding the heart- 

 tube. As yet, however, neither of the Cuvierian ducts opens 

 into the heart-tube. From the medial side of each Cuvierian 

 duct a few small capillaries are given off. Tliey run towards the 

 middle line and represent that portion of the anterior cardinal 

 vein which at a later stage connects the venae capitis medialis and 

 lateralis with the Cuvierian duct. (Compare PI. II. fip' 7 



A.o.r.) . 



Stage IV., therefore, possesses alieart in which the endothelial 

 tubes have fused antei-iorly and curvature has already com- 

 menced. Two complete aortic arches, an incomplete vena capitis 

 medialis, traces of a vena capitis lateralis, and Cuvierian ducts 

 are present. 



Stage Y. Perameles ohesula (10. viii. 03). 

 Macropus sp. 



The material for this stage consists of five similar embryos of 

 P. ohesula, three cut transversely and two longitudinally, and one 

 embryo of Onychogale frenata (? Macropus sp.) cut transversely. 

 In several respects, e. g. curvature of the heart, the Onychogale 

 embryo represents a slightly earlier stage than Perwmeles ohesula 

 (10. viii. 03). For purposes of description, however, it will be 

 convenient' to deal first with the Perameles embryos, as a wax- 

 plate reconstruction was made of the heart and anterior vessels 

 of embryo A (PL II. figs. 6-8). 



As regards general development, Perameles obesula {\Q . viii. 03) 

 shows only a slight advance on P. nasuta (IS) Stage III. The 

 brain has practically not changed ; the gut is in the same condition 

 except that the first visceral pouch is more extensive and closure 

 of the fore-gut has progressed back to slightly behind the auditory 

 vesicle. 



In the vascular system, however, we find a most marked 

 advance, the heart having assumed a definite form with ventricular 

 and auricular divisions recognisable. Figs. 6 to 8 illustrate the 

 model of the heart and anterior vessels in this stage. The gut, 

 included to form a building base, is coloured white, heart endo- 

 thelium and arteries red, veins and most of the capillaries blue, 

 myocardium yellow. The myocardium is left intact on the left 

 half of the model from the roots of the aortic arches back to the 

 level of the anterior end of the Cuvierian duct, but has been 

 omitted on the right side so that the whole of the endothelial 

 tube is here exposed to view. The capillaries surrounding the 

 gut are also left intact on the left side of the model, but on the 

 right have been omitted in order that the aortic arches might be 

 seen more clearly. Study of the actual sections shows that the 

 capillaries of the right side closely resemble those of the left. 



