(i BRANCHIAL 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 (1) the 

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

 branchial 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 pro- 

 cess. 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 leftside, 

 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 ob- 

 served, 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 growth push 

 the original clefts over to the same side as the mouth. Each of the fresh 

 clefts becomes divided into two, which form the permanent clefts of their 

 side. 



The gill slits at first open freely to the exterior, but during their 

 formation two lateral folds of the body wall, containing a prolongation 

 of the body cavity, make their appearance (fig. 4 A), and grow down- 

 wards over the gill clefts, and finally meet and coalesce along the 



FIG. 4. SECTIONS THROUGH TWO ADVANCED EMBKYOS OF AMPHIOXUS TO SHEW THE 



FORMATION OF THE PERIBRAHCHIAL CAVITY. (After Kowalevsty.) 



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; pp. body cavity. 



ventral line, leaving a widish cavity between themselves and the body 

 wall. Into this cavity, which is lined by epiblast, the gill clefts open 

 (fig. 4 B, br.c). This cavity which forms a true peribranchial cavity- 

 is completely closed in front, but owing to the folds not uniting 

 completely behind it remains in communication with the exterior by 

 an opening known as the atrial or abdominal pore. 



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. 



