EMBR YONIC DE VEL OPMENT. 



117 



a shallow depression in the floor of the 

 gut. This is the indication of the first 

 gill-slit which becomes perforated at 

 this point later. 



From this stage it is an easy tran- 

 sition to the stage which marks the 

 close of the embryonic and the com- 

 mencement of the lan>al period of 

 development. 



In the embryo shown in Fig. 64, the 

 mouth appears as an oval aperture placed 

 asymmetrically on the left side. At its 

 first origin it is relatively much smaller 

 than shown in the figure. A disc-shaped 

 thickening of the ectoderm appears on 

 the left side, in the region of the first 

 myotome. The subjacent endoderm 

 fuses with the thickening, and then the 

 centre of the disc becomes perforated, 

 and so the mouth is formed. 



The club-shaped gland has acquired 

 an opening to the exterior immediately 

 below the mouth, on the left side ; 

 while the body of the gland lies on the 

 right side. 



Behind the club-shaped gland on the 



a-n.. 



n.c 



Fig. 64. Stage in which the external apertures of 

 the body, prasoral pit, mouth, first gill-slit, and anus 

 have become perforated. Age about 36 hours. From 

 the left side. (After HATSCHEK.) 



al. Alimentary canal, an. Anus. b.c. Body-cavity. Fig. 64. 



ch. Notochord. end. Endostyle. gl. Club-shaped gland, 

 which has acquired an opening to the exterior on 



the left side below the mouth. g.s '. First primary gill-slit, m. Mouth, n.c. Xerve- 

 tube ; the neurenteric canal has closed up, but the nerve-tube still curves round 

 the hinder end of the notochord. np. Neuropore. p.o.c. Prasoral ccelom (right 

 head-cavity), p.p. Praeoral pit (left head-cavity), t. Provisional caudal fin. 



