DEVELOPMENT OF THE MEDULLARY TUBE, ETC. 



541 



of the blastopore, which thus becomes covered by a layer of ectoderm 

 (Fig. 281). The blastopore thus does not open externally but into the 

 neural canal ; this connection between the intestine and the neural 

 tube is long retained, being known as the neurenteric canal. [See 

 Kopsch, Xo. V. ; 



The medullary plate does not reach the most anterior end of the 

 embryo, but extends for about three-quarters of its length. At the 

 point where it stops, which lies somewhat in front of the anterior 

 edge of the first primitive segment, the medullary tubes retain an 

 external aperture which is at first wide but gradually narrows 

 later (neuropore, Figs. 285, 286, np). As we shall presently see, 

 the neuropore in Ampkioxtts 

 only closes in a very late stage 

 (Kupffer). The cells of the 

 medullary tube, like the other 

 ectodermal cells, carry rlagella. 

 These, which are long and ex- 

 ceedingly hue, project into the 

 lumen of the tube and are 

 directed backward. 



The development of the 

 medullary tube leads to a 

 pressing inward of the middle 

 part of the dorsal wall of the 

 entoderm - sac (Fig. 282 A). 

 This median swelling is accompanied by two latero-dorsal out- 

 growths of the entoderm-sac (Fig. 282 B, ink). These paired 

 longitudinal folds, the so-called mesoderm-folds, yield the material 

 which becomes the mesoderm and can be traced posteriorly as far as 

 to the neighbourhood of the two primitive mesoderm-cells,* although 

 the most posterior part of the folds is indistinctly marked off from 

 the rest of the entoderm-sac. Segmentation very soon appears in 

 the anterior region, the mesoderm-folds becoming cut up by trans. 

 \ eise indentations, into consecutive portions, the primitive segments 

 (Fig. 281, us', /is"). The segments which, owing to their origin, must 

 be regarded as archenteric diverticula, develop regularly from before 

 backward. We thus see, in Fig. 281 two, in Fig. 285 five, and in 

 Fi<r. 286 nine segments distinctly marked off. Later, the primitive 

 segments become completely cut oft' from the entoderm-sac, and they 



Fig. 2S3. —Transverse section through an 

 embryo of Amjahioxus with live well- 

 developed primitive segments (after 

 Hatschek. from O. Hertwig's Text- 

 book). 



[* See footnote, p. 539.— Ed.] 



