466 MISS K. M. PARKER ON THE 



margins lies the continuous pericardial cavity which shows a 

 marked increase in size as compared with the previous stage. 

 (Compare PL I. figs. 2 & 3, P.p.C.) This increase is greater in the 

 median anterior limb of the pericardium than in its lateral 

 portions. (Compare text-figs. 4 & 5.) 



The differentiation of the endothelial primordia of the heart 

 has progr-essed considerably and they now extend as actual tubes 

 even to the middle line of the pericardium. This fact is some- 

 what remarkable, as such a condition, in which the lateral heart- 

 tubes are actually in contact at their extreme cephalic apices 

 and diverge widely and abruptly from this point of contact, is 

 not found in any other stage. 



Some distance behind its cephalic extremity, the lateral heart- 

 tube gives rise to the first aortic arch (fig. 3, A 1), which follows 

 the antero-lateral mai'gin of the gut almost to the middle line 

 and there becomes continuous with the corresponding dorsal 

 aoi-ta, the two aortas being completely established in this stage. 



It will be vmnecessaiy to go into further details of the structure 

 of this embryo, as the sectional appearances found in it are 

 exemplified yet more clearly in the Perameles embryo to be 

 described next. It is, however, an important stage with regard 

 to the processes of growth and folding which bring about the 

 relations of the brain, gut, and pericardium Avhich are found in 

 subsequent stages. 



Stage III. Perameles nasuta (1 S). 



The material in this stage consists of two flat embryos with 

 widely open brain-plates and lateral heart-tubes. Embryo A has 

 eleven somites, the first being small and indistinctly limited; 

 embryo B has nine, the most anterior being indistinct here also. 

 The following description is based mainly on embryo A, which 

 was cut transversely. 



Total length of embryo A after partial flattening under cover- 

 glass, from the anterior margin of the brain-plate to the hinder 

 extremity of the primitive streak : 7*5 mm. 



Vascular ai-ea, 8*96 x 5*8 mm. in diameter. 



Text-fig. 6 represents a dorsal view of embryo A, while the 

 outlines of the gut and pericardium and the endothelial heart- 

 tube and aorta of the left side are given on PI. I. fig. 4. From 

 these figures it will be seen that the stage shows a considerable 

 advance in general development on the preceding. The head-fold 

 has progressed back as far as the region of the auditory pit. 

 The brain, though flat and widely open in the fore- and mid- 

 bi-ain regions, is deeply grooved in the hind-brain region. Lying 

 lateral to the brain-plate are two pairs of mesodermal masses, 

 one the maxillo-mandibular process, the other the hyoid arch. 

 Between these two arches is the first visceral pouch, and behind 

 the hyoid arch the second visceral pouch is already developed. 



