THE ASCIDIANS AND AMPHIOXUS II3 



section still makes up only the ventral and lateral walls of the gut tube, 

 now starts to grow inwards from each side towards the dorsal midline. In 

 doing so, it first undercuts the mesodermal strips, pushing them outwards 

 as grooves which eventually become completely cut off from the gut as 

 hollow sacks; and after pushing off the mesoderm, it then grows under 

 the chorda, excluding it also from the walls of the gut, which thus becomes 

 completely lined by endoderm. It should be noted that although this is a 

 convenient method of describing what happens, we do not actually know 

 that it is primarily the endoderm which pushes off the mesoderm, and 

 not the latter which actively withdraws itself; such questions could only 

 be definitely answered by an experimental analysis which has not yet 

 been made. It may well be the mesoderm which plays the active part 

 in its removal from the gut wall, since soon after this it undergoes a change 

 for which the endoderm can scarcely be made responsible; the long 

 hollow sacks of mesoderm become nipped off into a series of short, 

 square chambers, which lie in orderly pairs on each side of the chorda; 

 these are the somites. 



Only the first eight or nine pairs of somites are formed in this way as 

 outpushings from the gut wall. By the time they are laid down, the 

 blastopore, hidden under the ectodermal fold, is nearly closed. The fur- 

 ther growth in length of the larva is brought about by the prohferation of 

 all the material lying round this remnant of blastopore. This mass of 

 rapidly dividing cells grows out posteriorly, differentiating directly as 

 it does so into neural plate, notochord, somites and endodermal gut wall. 

 It has been suggested that in higher vertebrates too the more posterior 

 regions of the body are formed from a general prohferating mass in which 

 the embryological processes are by no means so clear-cut as they are in 

 more anterior regions; but we shall see (p. 263) that recent work makes this 

 unlikely. 



There is no need for our present purposes to follow the later history of 

 the Amphioxus larva in detail. It should perhaps be mentioned that the 

 mouth forms as an opening which breaks through into the gut near its 

 anterior end, while the anus is a similar opening nearer to the site of the 

 blastopore remnant, but somewhat anterior to it. The somites, from a 

 fairly early stage, expand laterally, growing round between the ectoderm 

 and endoderm till they fuse in the mid-ventral line, and thus provide 

 the gut with a complete mesodermal clothing. These lateral extensions 

 of the somites are, from the first, hollow like the somites from which they 

 come, and the space between their inner and outer walls is the origin of the 

 final body-cavity or coelom. If this cavity is traced back, it will be found 

 to be derived directly from the cavity of the gut or enteron; whence a 



