EMBRYONIC DEVELOPMENT. 



Growth of Free-swimming Embryo. 



Between 4 and 5 A.M. in the first morning of develop- 

 ment, i.e. at about the eighth hour, the embryo has reached 

 the stage represented in Fig. 62, and it now bursts through 

 the vitelline membrane and becomes free, swimming by 

 means of its cilia at the surface of the sea, or aquarium. 



The fact that Amphioxus has a free-swimming, ciliated 

 embryo is important as providing a general connecting 

 link between the Vertebrates and the Invertebrates, since 



arc- 



ftn.c 



ji.n.c 



Fig. 62. Embryo of Amphioxus at the stage at which it ruptures the follicle 

 and becomes free-swimming. 



A. Seen from above as a semi-opaque object. (After KOWALEVSKY.) 



B. Seen in sagittal (optical) section. (After HATSCHEK.) 



arc. Archenteron. m.p. Medullary plate. my.c. Myocoelomic pouches of 

 archenteron. p.n.c. Posterior neurenteric canal. 



the possession of a ciliated ectoderm is very common 

 among Invertebrate embryos, but entirely unknown among 

 the craniate Vertebrates. 



The medullary plate is now being closed off from the 

 outer surface. This is effected by the co-operation of two 

 factors. The ectoderm which bounds the medullary plate 

 laterally, grows over it, and simultaneously the ectoderm of 

 the posterior or ventral lip of the blastopore grows for- 

 ward over the medullary plate so as to shut in the blasto- 

 pore from the exterior (Fig. 62 A and B}. The archenteric 



