602 E. W. MAOBRIDE. 



the adult condition as a ciliated area on the inner side of the 

 prseoral hood. By the time that the head cavities have com- 

 menced to appear, notochord and nerve-cord have become well 

 advanced in their development. The notochordal fold has 

 become completely shut off from the gut, and is quite solid in 

 front, though still a groove behind, whilst the neural plate has 

 passed through the stage of being covered by a flap growing 

 from the adjacent ectoderm (figs. 13, 14, and 16), and has 

 become converted into a tube (fig. 17, b, c, and «?), still re- 

 taining an anterior opening, the neuropore (figs.l7,a, and 18,6). 

 The collar cavities have now become large thin-walled sacs; 

 the right extending by this time nearly to the mid-ventral 

 line; the left does not extend so far, but it still retains its 

 communication with the gut. This communication has by this 

 time become drawn into an exceedingly narrow tube (fig. 17, c, 

 neph.), and is in fact the rudiment of Hatschek's ne- 

 phridium. 



At the close of what we may term the embryonic development 

 — that is to say, at the end of the second day of development 

 — both collar cavities have undergone further changes. They 

 have extended forward at the sides of the notochord, above the 

 head cavities, to just beneath the neuropore (fig. 18, b); behind 

 they have nearly reached the mid-ventral line, the left being 

 more obliquely directed, as it has to pass over the area Avhere 

 the future mouth will be formed (fig. 19, b). When they have 

 reached the ventral line they extend backwards to a con- 

 siderable distance behind the first gill-slit, forming the ventro- 

 lateral angles of the body, and giving to the transverse section 

 a squarish appearance ventrally, which contrasts strongly with 

 the rounded appearance behind the point they extend to 

 (compare PI. 44, fig. 20, a and c). The inner walls of the 

 dorsal portions of the collar cavities become like the corre- 

 sponding parts of the somites converted into longitudinal 

 muscles, and constitute in fact the first myotomes; but during 

 early larval life, at any rate, the persistent cavity of this ''first 

 myotome^' on the right side remains in open and obvious 

 communication with the ventral part of the collar cavity 



