94 GENERAL DEVELOPMENT. 



myotomes, which nearly, though not quite, correspond in 

 number with the branchial pouches. 



The growth of the myotomes would seem, 

 as might be anticipated from their indepen- 

 dent innervation, not to be related to that of 

 the branchial pouches, so that there is a want 

 of correspondence between these parts, the 

 extent of which varies at different periods of 

 life. The relation between the two in an old 

 larva is shewn in fig. 47. 



The head of the larva of Petromyzon FlG . ^ EYE OF A LARVA 

 differs very strikingly in general ap- OF PETROMYZON NINE DAYS 



.. . AFTER HATCHING. 



pearance from that of the normal /. lens r. retina. 



Vertebrata. This is at once shewn The section passes through 

 by a comparison of fig. 43 with fig. 29. one side of the lens * 

 The most important difference between the two is due to the 

 absence of a pronounced cranial flexure in Petromyzon ; an 

 absence which is in its turn probably caused by the small 

 development of the fore-brain. 



The stomodaeum of Petromyzon is surprisingly large, and its 

 size and structure in this type militate against the view of some 

 embryologists that the stomodaeum originated from the coa- 

 lescence of a pair of branchial pouches. 



In the region of the trunk there is present an uninterrupted 

 dorsal fin continuous with a ventral fin round the end of the 

 tail. 



There is a well-developed body cavity, which is especially 

 dilated in front, in the part which afterwards becomes the 

 pericardium. In this region is placed the nearly straight heart, 

 divided into an auricle and ventricle (figs. 42 and 43), the latter 

 continued forwards into a bulbus arteriosus. 



The myotomes are now very numerous (about 57, including 

 those of the head, in a three days' larva). They are separated 

 by septa, but do not fill up the whole space between the septa, 

 and have a peculiar wavy outline. The notochord is provided 

 with a distinct sheath, and below it is placed a subnotochordal 

 rod. 



The alimentary canal consists of a narrow anterior section 

 free from yolk, and a posterior region, the walls of which are 



