GASTRULATION 



31 



Had the process been merely as stated the result would have 

 been a gastrula with a very wide mouth. But along with the 

 process of involution 

 there takes place an 

 active growth of the 

 lip or rim of the 

 gastrula. This 

 growth is especially 

 active anteriorly as 

 is shown by the fact 

 that mitotic figures 

 are most numerous 

 in this region and 

 become less and less 

 frequent towards the 

 posterior part of the 

 rim. The result is 

 that the original 

 mouth of the gas- 

 trula — the proto- 

 stoma — becomes 

 gradually en- 

 croached upon by 

 the gastrular lip — 

 the encroachment 

 being most marked 

 anteriorly so that 

 the opening becomes 

 not merely dimin- 

 ished in size but 

 also appears to shift 

 its position towards 

 what will become 

 the posterior end of 

 the embryo. It will 

 be noticed that what 

 really happens is not 

 a process of shifting 

 of the opening as 

 a whole, but rather 

 the persistence of 

 the hinder portion 

 of the opening 

 while its anterior 

 portion has disap- 

 peared. Such a remnant of the original protostoma may conveni- 

 ently be known by the special term blastopore. 



Expressed somewhat differently — the cavity of the gastrula has 



Fig. 18. — Illustrating the process of gastrulation in Amphioxus 

 as described by Cerfontaine. The second polar body marks 

 the neighbourhood of the apical pole. The individual 

 figures are viewed from what is seen later on to be- the left 

 side of the Amphioxus, the dorsal side being above and the 

 head end pointing towards the left side of the page. 



