THE EMBRYO AT THE TIME OF HATCHING. 61 



to be formed, lie a little way behind the anterior end of the 

 embryo, and the remaining ones are formed in succession from 

 before backwards as the embryo increases in length ; at the time 

 of hatching, two pairs of mesoblastic somites are usually present 

 (Figs. 24 and 26, CE). 



The notochord. The roof of the archenteron, between the 

 mesoblastic folds, is formed by a band of hypoblast cells lying 

 immediately below the neural plate, and in close contact with 

 this (Figs. 21 to 24). The cells composing this band, up to the 

 time of hatching, differ little if at all from the hypoblast cells of 

 the sides or floor of the archenteron ; but shortly after the time of 

 hatching, they undergo changes and become converted into the 

 notochord, the most important part of the skeleton of Amphioxus. 



Condition of the embryo at hatching. At the time of hatching, 

 which occurs about eight hours after fertilisation of the egg, the 

 embryo (Figs. 25 and 26) is ovoid in form, about twice as long as 

 it is wide, and in biilk about equal to the egg from which it was 

 developed (cf. Fig. 14, i). The epiblast is a single layer of short, 

 almost cubical cells, each of which bears a single flagellum, by 

 which the swimming of the embryo is effected. The neural 

 canal is roofed in for about the hinder third of its length ; in 

 front it opens to the exterior by a rather wide aperture, the 

 neuropore ; posteriorly, the neural canal communicates with the 

 archenteron through the neurenteric canal, the former blastopore. 

 The mesoblastic folds are present, and two pairs of mesoblastic 

 somites are already constricted off from their anterior ends. 



Immediately after working its way out of the egg membrane 

 the embryo swims to the surface of the water, and enters on the 

 second part of the embryonic period. 



Part II. From the hatching of the embryo to the formation of 

 the mouth : a period lasting from about twenty -four to twenty- 

 eight hours (cf. p. 48). 



The later stages of embryonic development consist chiefly in 

 further elaboration of the organs which are already established 

 at the time of hatching. The nervous system becomes more 

 complex; the mesoblastic somites increase considerably in 

 number, and undergo important changes whereby the muscular 

 and other systems are formed ; the notochord is definitely 

 established ; and at the close of the period the mouth and first 



