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, No. 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 Amphioxus 

 only closes in a very late stage 

 (KUPFFER). The cells of the 

 medullary tube, like the other 

 ectodermal cells, carry flagella. 

 These, which are long and ex- 

 ceedingly fine, project into the 

 lumen of the tube and are 

 directed backward. 



The development of the 

 medullary tube leads to a FIG. 283. Transverse section through an 

 pressing inward of the middle 



part of the dorsal wall of the HATSCHBK, from 0. HERTWIG'S Text- 



book). 

 entoderm - sac (Fig. 282 A). 



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

 growths of the entoderm-sac (Fig. 282 B, mk). 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. 

 verse indentations, into consecutive portions, the primitive segments 

 (Fig. 281, us , //*"). 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 

 Fig. 286 nine segments distinctly marked off. Later, the primitive- 

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



[* See footnote, p. 539. ED.] 



