482 



MANUAL OF ELEMENTARY ZOOLOGY 



14 A. 



after 



About 



hours 



14«. 



triploblastic animal are now present. Epiblast, hypoblast, 

 and mesoblast are known as the germ layers, and though, as 

 we shall see, they arise in different ways in different cases, 

 they are present at an early stage in the embryos of all Triplo- 

 blastica. Before these processes are complete, hatching takes 

 place by the throwing off of the vitelline membrane, and the 

 embryo becomes a larva. Until the formation of the mouth 

 the animal is sometimes called the "free embryo." 



„.*., , m .„ 3 Hatching takes 



place about 

 eight hours 

 fertilisation, 

 twenty-four 

 later the 

 mouth is formed as 

 a small opening, 

 which rapidly en- 

 larges, on the left 

 side of the forepart 

 of the body. At the 

 same time, by per- 

 foration from within 

 Fig. 361.— Vertical (140.) and horizontal outwards, the first 

 (141&. ) longitudinal sections of an embryo gill-slit is formed as 

 of Amphioxus at a somewhat later stage a, median ventral 

 than that represented by 13 (Fig. 360).— ODen ; n „ w v,: rh sh jf ts 

 After Hatschek. opening wnicn snms 



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 henceforward 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 mid-ventral line, and each 

 in turn shifts on to the right side. They are primary slits, 

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

 fourteen of these slits have been formed, another series 

 appears above them on the right-hand side. There are 



'-B7l(i. 



ach. 



Archenteron ; end., endoderm ; h.c, region 

 which will become the anterior median pouch, or 

 head cavity; m.s. 1-3, mesoblastic somites ; n.p., 

 neural plate ; nch., notochord ; tie., neurenteric 

 canal. 



