(2k) 



of tne Acti notro Ci:a. (i^ii_;.19) 



The above coi/aition contiiiueo until tne oval blastopore be- 

 comes smaller atiU rouna in outline ( Pi t,s . 20-20 ( e) ) which chaixj^e 

 is also accomFanied by nirther growth of the eiiteron in a post- 

 erior direction nntil it almost touches the end of the larva. 

 The cells of the tv/o lateral cords of mesoderm have now in- 

 creased in number, have arrai^ed themselves so as to enclose 

 a cavity, continuous with tno cavi ty of the anterior one de- 

 scribed above and have become attached both to the lateral ec- 

 todermal wall ard the lateral endodGrinaj. wail, CFit^, 20(b) ) 

 Anteriorly ti.is safe , which is now horse-shoe snape (Fi^c. 20(e) ) 

 is still only attached to the gaxit,lionic thiciceninc and tiic ven- 

 tral ectoderrral wall. ("Pig, 20) The conditions whicli I have 

 just descrihea arc not due to the siirinJcage of ti;e mesodermal 

 lining away from ti;e wall of the larva for the t raiiS parency of 

 tije livin^, larva maJces it possible to see tne fomation of the 

 mesodermal sac , I have followed this formation, stop by step 

 maiiy times in the liviii^^ gastrula and larva as well as in sec- 

 tions and surface mounts. 



As the anterior end of the larva bends fartiier vei.trally 

 and becomes a definite preoral lobe, the round blastopore as- 

 sumes the shape of an oval with its major axis transverse to the 

 long axis of the -larva. The posterior part of ti;e larva increas- 



