110 



EMBRYOLOGY. 



lib 



tup 



ck 



ik 



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



which the first primitive segment is being formed, 



after HATSCHEK. 

 ale, Outer, ik, inner, ink, middle germ-layer ; Jib, 



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 



coelenteron close to each other two small evagiiiations, 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 e vagina- 



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 coelenteron which is found 



between them and separating 



them (its limits marked by 



two Stars * * in figs. 69 and pig 70 _ Cross section of an Amphioxus embryo, 



70), and which lies under in which the fifth primitive segment is in 



. , ,, P ,1 in process of formation, after HATSCHEK. 



the middle of the medullary ak> Outer> ik> inner> mlc> mkldle germ . layer; mft 



groove, represents the funda- medullary plate ; ch, chorda ; *, evagination 



/ 7 7 .7 / 7 v " of the ccelenteron ; dh, intestinal cavity ; Ih, 



ment oj the chorda (ch), body-cavity. 



The primary inner germ- 



layer therefore has noiv 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 (Hi) and represent the 'middle germ-layer, and 



ak 



mp 



ch 



Ih 



ik 



