K. MITSUKUEI; 



I. INTRODUCTORY. 



In certain early stages of reptilian embryos, such as that given 

 in Fig. 1., PI. L, there is seen in the posterior part of tlie embryonic 

 shield a mass enclosed ^vithin the limbs of the horse-shoe shaped 

 blastopore. This mass exists in previous stages, but becomes 

 conspicuous at this period, through being distinctly circum- 

 scribed in front and at the sides by the dorsal opening of 

 the blastopore. A series of transverse sections through a Chelonia 

 embryo of this stage is given in Series I, PI. lY., and the section 

 f in that series g-oes throush the level of this mass. The 

 epiblast, which consists, in more lateral parts of the .section, of 

 columnar cells, dips down at the lips of the blastopore into the meso- 

 blast with which it fuses. Between the two lips of the blastopore 



