﻿74 Proceedings of the Royal Physical Society. 



a hepatopancreas — the pancreas forming a superficial layer 

 over part of its ventral surface. The spiral valve is fore- 

 shadowed by a spiral coiling of the gut rudiment. There is 

 a well-developed solid postanal gut, which eventually breaks 

 up and disappears. The hypochorda develops a distinct 

 " primary sheath," and its cells show vacuolation like those 

 of the notochord. 



Coelomic Organs. — The two nephrostomes which characterise 

 the functional pronephros belong' originally to metotic meso- 

 derm segments II. and V. Segment I. develops a tubule 

 rudiment which soon disappears ; segments III. and IV. 

 develop tubules which also gradually atrophy while those of 

 II. and V. are becomino- enlaroed. Behind segment V. a 

 number of doubtful tubule rudiments are found. In its later 

 stages the pronephros becomes very large, due to the great 

 growth of the anterior part of the archinephric duct and of 

 the two functional tubules. Separate glomeruli occur in 

 pronephric chambers B and E {i.e., the pronephric chambers 

 of segments II. and V.). Both pronephric tubules are still 

 visible in the 30 mm. larva. The mesonephric tubules 

 develop independently both of duct and of coelomic lining. 



Vascular System. — The cavity of the dorsal aorta is formed 

 by the fusion of vacuoles in masses of protoplasm derived 

 from the sclerotomes. The endocardium is apparently 

 mesodermal in origin. The blood-corpuscles make their 

 appearance quite suddenly in the already formed blood- 

 vessels : it is suggested that they are mesodermal cells set 

 free from their neighbours by the drawing in of cell pro- 

 cesses accompanying an epidemic of mitosis. During a 

 prolonged period only one pair of aortic arches are present — 

 those of the external gills (hyoidean). The ductus Cuvieri 

 are at first simply portions of the general vitelline network. 

 In its later stages the venous system becomes strongly 

 asymmetrical, the anterior end of the left posterior cardinal, 

 and the left inferior jugular becoming much reduced. 



Skeleton. — The material is too fragmentary to show details 

 of the early development of the skeleton; it may be noted, 

 however, that the skull in its early stages shows marked 

 resemblances to that of Amphibia, that a single pair of 



