460 



MISS K. M. PARKER ON THE 



recognisable as such were included in the model, a certain amount 

 of light was shed on the development and early relations of the 

 cardinal veins and aortic arches. The following account there- 

 fore deals not only with the development of the heart, but also 

 with certain facts relative to the early development of the 

 vessels of the head. 



Description of Stages. 



Stage I. Perameles obesula (1 Z), 



Dasyums viverrinus (7 '5 mm. vesicle). 



The material for this stage consists of four embryos of Pera- 

 meles obesula, one cut longitudinally and three transversely, and 

 sevei-al of D. viveri'inus. 



(a) Perameles obesula (1 Z). 



* Total length of embryo A in curved condition = 6-08 mm. 

 In this stage there is a flat brain-plate with an extensive neural 

 €i-est proliferation in the cranial region (text-fig. 1, N.C.P. (T.G.)). 



Text-figure 1. 



Peratiieles obesula (IZ). Dorsal view of embryo A. 



Av. Area vasculosa. L.M. Lateral mesoderm. N.C.F. (T.G.). Neural crest 

 proliferation (primordium of trigeminal ganglion). S. Somite. A, A\. Level 

 of section represented in text-fig. 2. 



There are fovir distinct somites {S.) : behind the fourth is a 

 Semitic thickening with an indistinct posterior limit, and in front 

 of the first, another imperfectly defined mass, representing 

 presumably a transitory first somite. 



* All measurements of embryos and certain descriptive details are taken from 

 Professor Hill's original notes and photographs of the material. 



