100 



T. NISHIKAWA. 



Cut 3. 



Croffi-section through the " crowing point ' 

 of the lateral line. Zeiss 4 BB. 



which, however, cannot be distinctly observed from outside. This is Seessel's 

 pouch, and is seen in sections in C & D ; it is in close contact with the 

 hypochorda. 



The anläge of the lateral line is clearly visible on either side of the body. It 



à 



is very narrow for the greater part of its length, and it stops short at about the 

 middle of the tail, where it is thickened and presents a club-shaped termination. 

 Throughout the greater part of the lateral line there is a lumen, which is slit- 

 shaped in cross-section, but at the 

 posterior extremity it is absent. In 

 the anterior part where the lateral 

 nerve is in close contact with the 

 anläge of the lateral line the lumen 

 opens to the exterior at several 

 points. Cut 3 is a cross-section through what may be called the "growing point" 

 of the lateral line. The backward growth of the club-shaped termination of the 

 lateral line is caused by the multiplication of the cells of the deeper part of the 

 superficial layer of the epiblast. 



The spiral valve of the intestine makes its appearance as a folding of the 

 intestinal wall. 



The embryo of 35 mm. presents no markedly different features from the one 

 just described. The club-shaped termination of the lateral line has only proceeded 

 nearer the tail end. 



The embryos of 43, 48, and 50 mm. all resemble in their general features. 

 The external gills are longer and the jaws are more conspicuous. Figs. 7 and 8 

 are two drawings of the front part of the embryo of 50 mm. It may be noticed 

 that the openings of the nasal sacs are no longer circular, as it was in the em- 

 bryo of 32 mm. The head is now much compressed dorso-ventrally. The 

 spiracles have changed considerably and are now seen as a pair of small pits. 

 The ducli endolymphatid and their external openings are clearly visible. The 

 second visceral clefts, or the first gill slits, tend to unite on the ventral side. 



The embryo of 60 mm. corresponds to Balfour's stage Q. The dorso- 

 ventral compression of the head has proceeded so far that its form is essentially 

 that of the adult. The spiracles are no longer visible on the outside ; the lower 

 jaw has grown forward, and the mouth has been reduced to a slit-like opening. 

 The flaps of the first gilt arches, or the opercular flaps have grown together on 



