472 



MISS K. M. PARKER OX TUE 



branches and is blind at both ends and not connected as yet with 

 any other capillaries. From these vessels the Cuvierian ducts 

 are later developed. 



In this stage we have, therefore, lateral heart-tubes which, 

 while they approach one another anteriorly, are widely separate 

 in the posterior region. Paired dorsal aorta? and the hrst pair of 

 aortic arches are developed, whilst traces of the second arch are 

 also present. This stage accordingly agrees in its general features 

 with the 84 days rabbit described by Bremer (1). In addition 

 to the heart and arterial vessels there are also present in the head 

 the first ti-aces of the venous system in the form of discounected 

 portions of the vena capitis medialis and the primordia of the 

 Ouvierian ducts. 



Text-figure 11. 



Ferameles namta (1 S, B.). Longitudinal section tlirougli somitos showing 

 dorsal oft'shoots from the dorsal aorta. 



B.A. Dorsal aorta. B.Br.D.A. Dorsal branch of sam.'. IJnd. Eu.lothelium 

 of longitudinal vessel. iS. Somite. 



STACrE TV. Perameles nasata (2 P). 



The material consists of four embryos, A and C cut transversely, 

 B and D longitudinally. 



Each of the four embryos has fifteen or sixteen somites. Tlie 

 neural tube is still unclosed throughout its length, but the folds 

 tire closely approximated in the hind-brain region. The mid- and 

 fore-brain segments are widely open as in "Stage III., but the 

 primary cranial flexure has occurred so that the fore-brain is 

 ■bent forwards and downwards (see text-tig. 15). 



The relations of the gut, pericardium, and heart are indicated 

 in PI. I. fig. 5. It is well to note at this point that the 

 ■outline of the brain-plate as indicated in figure 5 is not 

 strictly comparable with the corresponding line in fio-. 4 

 (compare text-figs. 12 and 15). The fact "that the primary 

 <!ranial flexure has occurred, renders it impossible to plot the 

 morphological anterior end of the brain in the same plane recon- 

 struction witli the hind-brain, gut, etc. The difterence in the 

 relations of the brain to the pericardium in the two stages may, 

 however, be judged by the position of the auditory ne'uromere 

 {A.X.\ which lies at the posterior margin of the pericardium in 



