STEUCTURE OP THE EXCRETORY OWOANS OF AMPHFOXUS. 199 
Part 4. — The Development of the Left Series of Nephridia in 
the Larva. 
For many years I have been trying to trace the develop- 
ment of the nephridia in Amphioxus. In 1902 I collected a 
large amount of material from the Pantano at Faro ; but ill- 
health prevented my working out the development on the 
living larva, and I failed to do so on the preserved speci- 
mens. It was not till last year that I was again able, in 
Helgoland, to study the living larva, and succeeded in 
tracing some stages of the development of the excretory 
organs. In the meantime Legros had been studying the 
same subject in Naples, and published anonymously a pre- 
liminary notice of his results a short time ago (16).^ 
In the present paper I shall not discuss in detail the first 
origin of the nephridia, but restrict myself to a description of 
the stages found in the larva with from ten to fifteen gill-slits 
of the left hand series, and no trace of the right hand series. 
These are the only stages which I have been able to study 
sufficiently in the living state. 
Fig. 30 gives a left side view of a young larva with eleven 
slits. The anterior gill-slits are still well on the right side, 
but the hinder slits are in or near the middle line. The 
future dorsal edge of each slit may, of course, at this stage 
be more ventral than the future ventral edge. The nephridia 
are seen as small rounded sacs near the posterior ventral 
corner of each slit. Every slit from the first to the last has 
such a nephridium. At this early stage there is no atrium, 
the slits have an internal margin of thick branchial epi- 
thelium, which is thrown into characteristic folds when the 
branchial muscles contract, while the external margin of the 
slit is formed by a thin fold of the body wall, acting as a sort 
' Tlirougli the kindness of M. Legros I have had the oi^portunity of 
examining Ins sections, and I cannot agree witli liis conclusions as to 
the origin of the nephridia from the ccelomic epithelium, nor as to the 
presence of inteimal openings. But I believe he has modified his views 
considerably on these points since the publication of the note. 
