72 AMPHIOXUS. 



and extends along the whole length of the under surface of the 

 intestine. The anterior end of the aorta, on reaching the level of 

 the second somite, turns upwards, and runs obliquely forwards 

 along the right side of the pharynx, passing dorsal to the first 

 gill-cleft, and ending in close relation with the club-shaped gland. 



(3. Structure of the Embryo at the Close of the Embryonic 

 Period. 



The general appearance of the embryo at this stage is 

 shown in Fig. 34. The embryo has a total length of about 

 1*3 mm., and is of a glassy transparency in all its parts and 

 organs, owing to the complete absorption of the yolk granules 

 originally present in the egg. It is widest about the level 

 of the mouth, in front of which it tapers rapidly, ending in a 

 sharply pointed snout. The hinder part of the body tapers 

 very gradually, and ends in a thin vertical fin of rather larger 

 size than is shown in the figure. 



The embryo swims actively, by alternating contractions of the 

 myotomes of the two sides of the body. Of these myotomes 

 there are fifteen pairs present ; the myotomes of the first pair 

 are opposite each other, those of the next two or three pairs are 

 placed more or less obliquely, and behind the fourth pair the 

 myotomes alternate regularly along the two sides of the body. 

 The first pair of myotomes give off anterior prolongations, which 

 extend along the sides of the notochord to the tip of the snout, 

 and by their contractions bend the snout freely from side to 

 side. Each muscle fibre is formed by elongation of a single 

 cell, and the majority of the fibres show more or less evident 

 transverse striation. The alimentary canal is divided into an 

 anterior, dilated, pharyngeal region, lying opposite the first two 

 myotomes ; and a posterior, cylindrical, intestinal region which 

 extends to the anus. In connection with the pharyngeal region 

 are the mouth, the first gill-slit, and the club-shaped gland ; 

 there is as yet no trace of the liver. 



The nervous system consists of a neural tube, with proper 

 walls of its own, extending the whole length of the back of the 

 animal, just above the notochord. The neural tube opens to the 

 exterior at its anterior end through the neuropore, immediately 

 behind which the tube presents a slight dilatation or ' brain.' The 

 posterior end of the neural tube (Fig. 34, ne) bends downwards 



