SECOND PERIOD OF DEVELOPMENT. 59 



partly reabsorbed the moisture found in the segmenta- 

 tion cavity. The mechanical side of the process is 

 also thereby explained to us. The hypoblast cells play 

 a more active part, the epiblast cells remaining pas- 

 sive during the whole process and forming a spherical 

 covering. 



By this time, owing to the first enlargement of the 

 hypoblast cells, which take a more columnar form 

 (Fig. 21), the flattening of the lower pole begins. 

 Further, owing to the diminution of the fluid in the 

 segmentation cavity, which we ascribe to an action of 

 the hypoblast cells, this flat surface is bent inwards, 

 since it offers less resistance to such bending than the 

 convex epiblast cells. The continual diminution of 

 the moisture in the segmentation cavity, aided as it 

 is by the change in form of the enlarging hypoblast 

 cells, requires continuous invaginatioii. The enlarge- 

 ment of the hypoblast cells makes it possible that 

 they, which originally occupy a relatively smaller sur- 

 face, form the whole lower stratum of the flat convex 

 gastrula stage. The extension of this inner surface is, 

 however, still always to be considered as far less than 

 that of the outer one which is formed of the epiblast 

 cells. 



