THE LATER EMBRYONIC DEVELOPMENT. 71 



groove close to form a tube, which splits off along its whole length 

 from the mesenteron, but remains in close contact with this. 

 The limb of the tube which lies at the right side of the mesen- 

 teron expands slightly to form the club-shaped gland (Pig. 36, 

 GL) ; the rest of the tube forms a slender duct, which passing 

 across the body, under the mesenteron, to its left side (Fig. 36, 

 Gd), acquires an opening to the exterior just below the anterior 

 border of the mouth, as soon as this latter is formed. The 

 further development of the club-shaped gland will be described 

 in the section dealing with the larval stages. 



c. The mouth. At the close of the embryonic period, a disc- 

 like thickening of the epiblast forms on the left side of the 

 head, opposite the first somite but ventral to its lower edge. 

 The hypoblast of the mesenteron fuses with this patch of epi- 

 blast, and the mouth is formed as a perforation in the middle of 

 the fused patch. The mouth is at first a minute circular aper- 

 ture, but it rapidly increases in size, and at the end of the embry- 

 onic period is a large oval opening (Fig. 36, o), with a slightly 

 thickened border, on the left side of the head. 



d. The first gill-slit. Simultaneously with the formation of 

 the mouth, a slight depression of the hypoblast of the ventral 

 surface of the mesenteron appears, opposite the second pair of 

 somites ; this fuses with the epiblast, and then, by perforation, 

 an opening is formed which is the first gill-slit (Fig. 34, l). 

 The perforation is formed from within outwards : the gill-slit is 

 at first very small, and situated in the mid-ventral wall ; but it 

 soon enlarges, and as it does so shifts upwards to the right side 

 of the body (Fig. 36, hk i). Like the mouth, it is bordered by 

 long cilia. 



e. The anus. This is formed shortly after the mouth and the 

 first gill-slit (Pig. 34, u). It is at first much nearer the hinder 

 end of the body than in the adult, and is placed slightly to the 

 left of the median plane. 



5. The Blood-vessels. 



The development of the blood-vessels in Amphiosus has been 

 but very imperfectly studied. The first vessel to appear is said 

 to be the ventral or cardiac aorta, which is developed in a longi- 

 tudinal strip of mesoblast, formed by fusion of the ventral edges 

 of the somites of the two sides along the mid-ventral line, 



