110 



EMBRYOLOGY. 



Fig. 69. Cross section of an Amphioxus embryo, in 



which the first primitive segment is being formed, 



after HATSCHEK. 

 at, Outer, ik, inner, ml, middle germ-layer ; hb, 



epidermis ; mp, medullary plate ; ch, chorda ; 



*, evagination of the ccelenteron. 



is comparable with a siphon. The upper arm, which is the neural 

 tube, continues, for a time, to open to the outside world at its 



anterior end. The bent por- 

 tion of the siphon, or the 

 blastoporic region, by means 

 of which the neural and the 

 intestinal tube are united, is 

 called canalis neurentericus 

 (fig. 68 en), a structure which 

 we shall again encounter in 

 the development of the re- 

 maining Vertebrata. 



Simultaneously with the 

 neural tube are developed 

 the two middle germ-layers 

 and the chorda dorsalis (figs. 

 69 and 70). At the front 

 end of the embryo there 

 arise in the roof of the 



ccelenteron close to each other two small evaginations, the body-sacs 

 (mk), which grow dorsally and laterally at either side of the 

 curved medullary groove. 

 These are slowly enlarged, 

 since the process of evagina- 

 tion progresses from the an- 

 terior toward the posterior 

 end of the larva, and finally 

 reaches the blastopore. The 

 narrow strip of the wall of 

 the ccelenteron which is found 

 between them and separating 

 them (its limits marked by 

 two stars * * in figs. 69 and fig 70 _ Crogs 

 70), and which lies under 

 the middle of the medullary 

 groove, represents the funda- 

 ment of the chorda (ch). 



The primary inner germ- 

 layer therefore has now undergone division into four different parts : 

 (1) the fundament of the chorda (ch), (2) and (3) the cells (mk) which 

 line the two body-sacs (Ih) and represent the middle germ-layer, and 



of an Amphioxus embryo, 

 in which the fifth primitive segment is in 



medullary plate ; ch, chorda ; *, evagination 

 of the ccelenteron ; dh, intestinal cavity ; Ih, 

 body-cavity. 



