382 



AN INTRODUCTION TO ZOOLOGY 



growing over it, the neural plate has become curved downwards 

 in the middle, thus leaving a small space under the superficial 

 ectoderm. Owing to the way in which the lower lip of the blastopore 

 grows upwards it closes the opening of the archenteron to the outside, 

 but leaves a small aperture, the neurenteric canal, which puts the 

 archenteric cavity in communication with the space above the 

 neural plate. Eventually this plate becomes more and more folded 

 until, finally, its two edges meet and fuse in the mid-dorsal line and 

 so forms the neural tube, which is the forerunner of the central 

 nervous system. It will be seen then that right from the very 

 beginning the central nervous system is hollow, dorsal in position, 

 and represents a specialised insinking of part of the original ectoderm, 



jnes. 



net 



FIG. 129. Amphioxus, later cleavage. From Bourne, after Hatschek. 



N., sagittal section of an embryo with three mesoblastic somites ; O., horizontal section of the 

 same embryo ; P., sagittal section of an older embryo ; Q., sagittal section of an embryo with nine 

 pairs of mesoblastic somites ; R., horizontal section of an embryo of the same age showing the 

 origin of the head-cavities ; ach., archenteron ; mes., mesoblastic somites ; n.c., neural canal ; 

 nee., neurenteric canal; np., neuropore ; I.-IX., the several pairs of mesoblastic somites; nch., 

 notochord. 



all important points. The neuropore and neurenteric canal remain 

 open for some time, but finally both close. 



While the formation of the neural tube has been taking 

 place a series of changes have occurred in the archenteron. At an 

 early stage it can be seen that the cells of the dorsal portion of the 

 entoderm are smaller than those on the sides and floor of the cavity, 

 and these dorsal cells become arranged in three longitudinal grooves, 

 of which the median is first to appear. By the time the neural plate 

 is roofed with ectoderm these grooves have become well marked, 

 and in transverse section present the appearance of three pocket- 

 like outbulgings from the archenteric cavity. The median of them 

 is quite a narrow groove. During further growth its sides approxi- 

 mate and their cells interlock, and, finally, about the same time as 



