GASTRULATION 



625 



off of the vitelline membrane, and the embryo becomes a larva 

 (P- 345)- Until the formation of the mouth the animal is sometimes 

 called the ' free embryo '. 



,n.p. 



^.-nch. 



- -enl. 



.n.c. 



n.c. 



m. pi." 



Fig. 483. — Transverse sections of embryos of Branchiostoma at successive stages 

 later than that represented by Fig. 481. — After Hatschek. 



ent., Gut or enteron ; ni.pl., muscle plate ; m.s., mesoblastic somite ; myoc, myocoele ; n.c, nerve cord 

 n.p., neural plate ; nch., notochord ; sop., somatopleure ; splc, splanchnocoele ; spp., splanchnopleure. 



LARVA 



Hatching takes place about eight hours after fertilisation. 

 About twentv-four hours later the mouth is formed as a small 

 opening, which rapidly enlarges, on the left side of the forepart 

 of the body. At the same time, by perforation from within out- 

 wards, the first gill slit is formed as a median ventral opening 

 which shifts upwards to the right side of the body. As we shall 

 see, this slit belongs to the left side of the body of the adult. The 

 anus is formed shortly after the first gill slit, much nearer the 

 hind end of the body than it is in the adult. Development hence- 

 forward takes place more slowly, the animal becoming adult in 

 about three months. We will consider first the external features 

 of its metamorphosis. More gill slits are formed in the midventral 

 line, and each in turn shifts on to the right side. They arc primar\- 

 slits, and each except the first acquires a tongue-bar. When four- 

 teen of these slits have been formed, another series ajiix^irs 

 above them on the right-hand side. These are eight in number. 



