208 ARTHUE WILLET. 



The formation of the tongue-bars commences very early iu 

 the secondary slits^ but not in the primary slits until they are 

 well round to the left side. The first slit on each side remains 

 simplCj and does not form a tongue-bar. 



As soon as all the primary slits that are going to close 

 have done so, the larva enters upon the so-called critical 

 period. At the end of this period fresh slits begin to form 

 normally on both sides, and continue to do so throughout life ; 

 these are called the tertiary slits in order to distinguish them. 



Thus the critical period may be defined as the period in 

 which the slits are arranged regularly — an equal number of 

 pairs — in two rows, right and left ; but all those on the left 

 side are primary (in the sense in which the term is here 

 employed), and all those on the right side are secondary. 



M 2. TheEndostyle. 



The forecast of the endostyle appears in the embryo at a 

 very early stage, namely, shortly after the formation of the 

 club-shaped gland in front of which it lies. It consists of a 

 patch of modified columnar hypoblastic epithelium, bending 

 obliquely backwards and folded forwards upon itself, and 

 lying on the right side of the mouth-cavity in the region of 

 the first myotome. 



The upper arm of the endostyle is at first much shorter than 

 the lower arm ; the former becomes the right half of the adult 

 endostyle, and the latter becomes the left half. 



The endostyle retains its original shape and its anterior posi- 

 tion until some time after the first appearance of the secondary 

 slits. 



At Stage IV it begins to fall away, as it were, from its 

 oblique position — the evidence of this being found in the fact 

 that in this stage it extends a short distance beyond the club- 

 shaped gland, so that the latter, instead of lying behind it, lies 

 upon it (fig. 5). Although the endostyle has thus grown 

 backwards, it has done so in bulk — that is to say, it has bodily 

 shifted its position without increasing in length. 



In Stage V, in addition to a further change of position, it 



