The Pronephros of Seyllium canicula. 241 
Embryo K. 
10 mm long. Sectioned and mounted upon 7 slides having 105, 
105, 105, 106, 105, 105, 105 sections respectively. 
Left side Right side 
2,25 —2,56 Ear 2,20— 2,48 
2,92— 2,102 I 2,86 —2,95 
2,103—3,10 IT 2,96—3,4 
3,12— 3,23 II 3,6—3,19 
3,25 —3,35 IV 3,20—3,32 
3,37— 3,48 M 3,33—3,44 
3,49—3,58 VI 3,45—3,56 
3,60— 3,68 VII 3,57— 3,66 
3,69—3,78 VIII 3,67—3,76 
3,79— IX 3,77 — 
The first four gill-clefts on each side are open, and the fifth 
cleft is nearly open. 
On the left side the front end of the pronephrie structure con- 
sists of an open groove of the coelomic epithelium from 3,64—3,70. 
This is obviously the first nephrostome and is situated in the post- 
erior region of the seventh somite. The groove is closed, but has 
a solid connection with the coelomic epithelium in 3,71 to 3,74, 
whilst in and after section 3,75 the pronephric duct has neither an 
opening into, nor a solid connection with, the coelom. Hence it 
appears that the only opening to persist as the Müllerian duct 
coelomic opening on the left side is the first nephrostome, and the 
tubule becomes the extreme anterior end of the Müllerian duct. In 
embryo J the nephrostome of this tubule was not confined to the 
seventh segment, but extended into the anterior region of the eighth 
segment also: that part of the nephrostome which extended into 
the eighth segment in the last-mentioned embryo is now closed up, 
while the second nephrostome, present in the posterior region of 
segment VIII in embryos I and J has now completely atrophied. 
On the right side the first nephrostome is in the form of an 
open coelomic groove from 3,64 to 3,69. The groove is closed, but 
has a solid connection with the coelomic epithelium in 3,70. In and 
after 3,71 the duct is quite free from the coelomic lining. From a 
comparison with the condition in embryos I and J it will be seen 
that it is the first nephrostome which persists, viz. that which ex- 
