202 ARTHUR WILLEY. 



ciliated tract of the prseoral pit is now beginning to alter its 

 shape — becoming constricted in the middle. This constriction 

 is carried still further in the next stage (fig. 15), and, as is 

 already known, the ciliated tract subsequently grows out into 

 several lobes, which give its characteristic appearance to the 

 Rader-organ. 



The most important feature in fig. 14 is the endostyle, which 

 has increased considerably in length, reaching to the fourth 

 slit. Its double nature is very apparent; the left (lower) half 

 lying, as before mentioned, at a much deeper focus than the 

 right (upper) half, so that the former only is seen from the 

 left side (fig. 13). 



In the larva represented in fig. 14 there were eight slits on 

 the left side, and the ninth was seen to be in course of closure 

 in the mid-ventral line. 



We are now approaching the period in the larval develop- 

 ment in which there are an equal number of slits on both sides 

 of the body, all those on the left side being primary and all 

 those on the right side secondary. 



After a long interval, during which the gill-slits increase in 

 vertical height and the endostyle extends further backwards, 

 fresh tertiary gill-slits form behind on each side in a manner 

 characteristic of later growth, and continue to do so through- 

 out life. 



The stage, then, with an equal number — seven to nine pairs 

 — of primary and secondary slits, and before the formation of 

 any tertiary slits, may well be called ''the Critical Stage."^ 



As a general rule the critical stage is characterised by the 

 presence of eight pairs of slits, namely, eight primary slits on 

 the left side and eight secondary slits on the right. 



Stage VIIL— Figs. 15—17. 



During this stage the critical stage, as defined above, is 

 firmly established, and in fact may be said to commence as soon 



1 The remarkable fact that the number of pairs of gill-slits in the critical 

 stage agrees approximately with the full typical number of the craniate gill- 

 clefts should not be lost sight of. 



