1 68 EMBRYOLOGICAL TYPES 



forms as a perforation just beneath the neurenteric canal, 

 which becomes closed and obliterated. Behind and dorsal to 

 the anus the tail begins to grow back. 



The cells lining the cavity of the gut become ciliated, and 

 the splanchnic layer of coelomic epithelium surrounding them 

 gives rise to smooth muscle-fibres. The liver grows out as a 

 diverticulum from the gut on the right side. 



The origin of the structures of the pharynx is peculiar and 

 complicated by the extraordinary asymmetry which the larva 

 shows. A structure is formed by the downgrowth of a groove 

 from the front of the floor of the gut, and is converted into a 

 tube which eventually opens into the gut on the right side, and 

 to the exterior a little to the left of the midventral line. This 

 is the so-called club-shaped gland, which is regarded as the 

 first gill-slit of the right side. The first gill-slit of the left side 

 arises ventrally by a perforation between the gut and the 

 ectoderm, and it moves up the right side of the body, opposite 

 the mouth. Behind this slit, about a dozen more are formed 

 ventrally, and likewise move up the right side, although they 

 are destined to become the left gill-slits eventually. This 

 series is known as the primary gill-slits. These slits corre- 

 spond with the segmentation of the body at this stage ; but 

 this correspondence is lost later on. 



The definitive gill-slits of the right side, or secondary 

 gill-slits, arise later than the primary, and above them on the 

 right side to the number of eight. The most anterior secondary 

 slit corresponds to the second primary slit, which is what would 

 be expected if the club-shaped gland is really the first right 

 gill-slit, corresponding to the first primary gill-slit. 



In front of the club-shaped gland, there arises a thickening 

 of the wall of the gut consisting of a strip of ciliated and 

 glandular cells. This is the rudiment of the endostyle. It 

 becomes V-shaped with the apex pointing backwards, and this 

 apex grows backwards as a double strip along the wall of the 

 pharynx on the right side above the primary slits and below 

 the secondaries. It is as if the morphologically midventral 

 line of the larva in the region of the pharynx were displaced 

 up on to the right side. Soon the primary slits move round to 



