CEPHALOCHORDA. 



these are being formed, remains cylindrical, but shortly after- 

 wards it becomes pointed at both ends, and the caudal fin 

 appears. The fine cilia covering the larva also become replaced 

 by long cilia, one to each cell. The mesenteron is still completely 

 closed, but on the right side of the body, at the level of the front 

 end of the mesenteron, the hypoblast and epiblast now grow 

 together, and a perforation becomes formed through their point 



/v 



VVJl 'IL Jl 



br.c 



FIG. 4. SECTIONS THROUGH TWO ADVANCED EMBRYOS OF AMPHIOXUS TO 



SHEW THE FORMATION OF THE PERIBRANCHIAL CAVITY. (After Kowalevsky.) 

 In A are seen two folds of the body wall with a prolongation of the body cavity. 

 In B the two folds have coalesced ventrally, forming a cavity into which a branchial 

 cleft is seen to open. 



mes. mesenteron ; br.c. branchial cavity; //. body cavity. 



of contact, which becomes the mouth. The anus is probably 

 formed about the same time if not somewhat earlier 1 . 



Of the subsequent changes the two most important are (i) 

 the formation of the gill slits or clefts ; (2) the formation of the 

 peribranchial or atrial cavity. 



The formation of the gill slits is, according to Kowalevsky's description, 

 so peculiar that one is almost tempted to suppose that his observations were 

 made on pathological specimens. The following is his account of the 

 process. Shortly after the formation of the mouth there appears on the 

 ventral line a coalescence between the epiblast and hypoblast. Here an 

 opening is formed, and a visceral cleft is thus established, which passes to 

 the left side, viz. the side opposite the mouth. A second and apparently a 

 third slit are formed in the same way. The stages immediately following 

 were not observed, but in the next stage twelve slits were present, no longer 

 however on the left side, but in the median ventral line. There now appears 

 on the side opposite the mouth, and the same therefore as that originally 

 occupied by the first three clefts, a series of fresh clefts, which in their 



1 The lateral position of the mouth in the embryo Amphioxus has been regarded 

 as proving that the mouth represents a branchial cleft, but the general asymmetry 

 of the organs is such that no great stress can, I think, be laid on the position of the 

 mouth. 



