474 



MISS K. M. PARKER ON THE 



are prolonged into the lateral gut-folds and pass gradually into 

 vitelline veins. 



The heart as a whole is somewhat asymmetrical (see fig. 5), 

 being curved over to the right side of the embryo. 



The aortic arches, two of which are developed, arise from the 

 median bulbus arteriosus. The endothelial heart-tube bifurcates 

 in front, and each half runs forwards and slightly outwards as a 

 relatively wide vessel situated between the two layers of the 

 splanchnopleure. These vessels, which constitute the first aortic 

 arches, run forwards and outwards in a course similar to that 

 of the same vessels in the next stage (compare figs. 5 and 6). 

 Anteriorly they loop round the gut to join the dorsal aortse. 

 From the anterior convexity of this first aortic arch are given off 

 capillaries which form a network surrounding the primary optic 

 vesicles. Prom the lateral margin of the fused tip of the heart 

 is given oflf on each side a small vessel which runs outwards and 

 backwards, loops round the gut, and constitutes a continuous 

 though slender second aortic arch. 



Text-figure 13. 



Ferameles nasuta (2 P, A). Transverse section through bulbus arteriosus. 



Bnd. Endothelium. Gr. Gut. My. Myocardium. P.O. Pericardial cavity, 

 Jtt.A. Root of aortic arch. 



The vena capitis medialis, which was just recognisable in the 

 preceding stage, is now considerably further developed. It is 

 represented by an irregular and not perfectly continuous series 

 of capillaries, situated dorsal to the aorta on either side of the 

 medullary tube, medial to the cranial ganglia. These capillaries 

 are connected by very fine sprouts with the dorsal aortfe (text- 

 fig. 14, V.C.M.) ; ventro-lateral to the auditory vesicle and lateral 

 to the nerve-roots, there is another line of scattered capillai-ies 

 connected with the vena capitis medialis. These are the first 

 traces of the vena capitis lateralis. Portions of the vena capitis 

 medialis can be traced in the region of the lateral heart-tubes as 

 far back as the Cuvierian ducts {i. e., the region of the third 



