86 WILLIAM BATESON. 
gill-slits this thickening has considerably increased, and in the 
contracted condition of the body is a very marked structure 
(fig. 2, op.). As development proceeds, this thickening in- 
creases, and at the same time grows backwards in the lateral 
regions until (five gill-slits) it has covered half the first gill- 
slit. The degree of contraction of the body of course alters its 
relations, but in the extended condition in adult specimens its 
posterior margin is about on a level with the fourth gill-slit. 
As then this structure is a process of the body wall which 
forms an opercular fold over the foremost gills, it appears 
reasonable to institute a comparison between it and the atrial 
walls of Amphioxus, &c. It will therefore be alluded to as the 
operculum, and the cavity between it and the body wall as 
the atrial cavity. 
As previously described, the first gill-slit on its appearance 
is a simple circular pore. In this condition it remains for 
some days, but gradually, on about the tenth day, its form 
changes owing to the growth of a process from the dorsal 
margin of the pore, which renders the aperture somewhat 
kidney-shaped (fig. 1). Whilst this process is continuing the 
second gill-slit appears as another circular pore, similar to the 
original aperture of the first gill-slit. I was unable to discover 
any priority in the appearance of the gill-slit of either side in 
particular, but incline to believing that they appear syn- 
chronously on the two sides of the body, as previously 
mentioned. 
After the appearance of the first gill-slit a definite series of 
changes is undergone, and these form a definite period in the 
history of the development of the animal, which may be con- 
sidered as closing with the formation of the second pair of gill- 
slits. 
At this point it may be well to recapitulate these changes. 
The embryonic life terminates with hatching, and the free 
larva with one pair of gill-slits escapes, moving about in the 
mud by means of cilia. 
Before the second pair of gill-slits appear, it has undergone 
the following changes : 
