128 



INTRODUCTION. 



ms 



hypoblast cells necessitates a 

 modification of the normal pro- 

 cess of embolic invagination, 

 and causes another process to 

 be substituted for it, viz. the 

 growth of the epiblast cells as 

 a thin layer over the hypoblast. 

 This process (fig. 57) is known 

 as epibolic invagination. 

 The point where the complete 

 enclosure of the hypoblast cells 

 is effected is known as the blas- 

 topore. All intermediate con- 

 ditions between epibolic and 

 embolic invagination have been found. 



In delamination, when the segmentation is not uniform, or 

 when a solid morula is formed, the differentiation of the epiblast 

 and hypoblast is effected by the separation of the central solid 

 mass of cells from the peripheral cells (fig. 58 A). 



FIG. 57. TRANSVERSE SECTION 

 THROUGH THE OVUM OF EUAXES 

 DURING AN EARLY STAGE OF DEVELOP- 

 MENT. (After Kowalevsky.) 



ep. epiblast ; ms. mesoblastic band ; 

 hy. hypoblast. 



FlG. 58. TWO STAGES IN THE DEVELOPMENT OF STEPHANOMIA PICTUM. 



(After Metschnikoff.) 



A. Stage after the delamination. ep. epiblastic invagination to form pneuma- 

 tocyst. 



B. Later stage after the formation of the gastric cavity in the solid hypoblast, 

 po. polypite ; t. tentacle ; //. pneumatophore ; ep. epiblastic invagination to form 

 pneumatocyst ; hy. hypoblast surrounding pneumatocyst. 



