194 ARTHUK WILLEY. 



This table will be intelligible if oue compares what has been 

 said above with reference to the first secondary slit with the 

 position of the latter as given in the fourth column. In 

 No. 7 it will be noticed that the first two secondary slits 

 were late in appearing. No. 6 in the above table was 

 obviously aberrant, but is interesting as exhibiting a hastening 

 of the development of the full number of secondary slits. It 

 was also slightly abnormal, since, although the ninth slit was 

 well open, the eighth was only present as a thickening. 



In all cases the eventual first secondary slit arises above and 

 between the second and third primary slits. 



The trifling confusion which may attend this description is 

 unavoidable. The straightforward course of events, unham- 

 pered with variations, will be given in the summary. 



Stage IV.— Figs. 5, 6, and 26. 



This is a well-marked stage, characterised by a general 

 increase in size of the secondary slits, accompanied by the 

 dipping downwards or bending inwards of the dorsal wall in 

 the largest of them — namely, Nos. 3 to 5 inclusive. The 

 fusion of the down-growth from the dorsal wall with a small 

 up-growth from the ventral wall of the slit (which, however, 

 does not occur in this stage) results, as is well known from 

 the figures of Kowalewsky and others, in the formation of the 

 so-called tongue-bar of the slit. 



The bending under of the primary slits from the right to 

 the left side has now proceeded much farther ; and the first 

 primary slit is very much reduced in size. In the next stage 

 we shall see that it closes up and ultimately disappears without 

 leaving a trace. 



In fig. 5 the thirteenth primary slit is seen in course of 

 closure — that is to say, in a ventral view it would present the 

 appearance shown in the case of the twelfth slit in figs. 9 and 

 10. There are seven secondary slits, the first being between 

 the second and third primary slits. 



The atrium is now completely closed anteriorly. 



The most striking and unforeseen characteristic of this 



