KLEINENBERG ON DEVELOPMENT OF LOPADORHYNCHTTS. 537 



the seta-sac, gives rise to the ventral cirrhus. At a later 

 stage the neural plates of the opposite sides of the body become 

 connected together by a series of transverse commissures. 

 These commissures are developed coincidently with the elonga- 



te 



Fig. 1. — Transverse section of a young larva of Lopadorhynchus, showing 

 the formation of the muscle-plate from the ectoderm. Ec. Ectoderm, end. 

 Endoderm. M.P. Muscle-plate. S.O. Sense organ. 



tion of the body in such a way that for each pair of seta-sacs a 

 single commissure is developed. Each pair of seta-sacs with 

 its transverse commissure represents a single somite. In 

 addition to the transverse commissures, longitudinal fibres are 

 developed towards the median edges of each neural plate; these 

 put the ganglionic centres of the ventral chain into communi- 

 cation with one another, and form the longitudinal connectives. 

 From their mode of formation the connectives are necessarily 

 paired structures ; subsequently they become intimately united 

 together ; but their paired structure is always distinguishable. 

 At first both commissures and connectives lie directly upon 

 the ectoderm, but subsequently become separated from it. 

 The foundations of the dorsal and ventral cirrhi are divided 

 into proximal and distal portions. The latter form bundles of 



