vi PREGNANCY PARTUEITIONPUEEPEEIUM 213 



mesoderm or mesoblast, which in like manner gradually extends 

 towards the equator, eventually reaching the inferior pole of the 

 ovum. 



The mesoderm afterwards subdivides into two layers, one 

 superficial, which remains in contact with the ectoderm and is 

 therefore called parietal, and the other remaining in contact with 

 the endoderm and therefore termed visceral. The parietal layer 



PIG. 76. Sections of mammalian ovum to show the formation of blastocyst. z.p., zona pellucida ; 

 ect., peripheral stratum ; ent., internal cellular mass, embryonic bud. 



forms the so-called somatopleure, the visceral layer constitutes the 

 splanchnopleure. The term coelom or body-cavity is applied to 

 the space included within these layers. The division of the 

 mesoderm into two layers takes place throughout the whole 

 blastodermic vesicle ; it begins in the embryonic area and proceeds 

 towards the inferior pole of the ovum as far as the extension of 

 the mesoderm. The coelom may then be distinguished as internal, 

 or intra- embryonic, and external, or extra -embryonic. Before, 

 however, we examine the successive developments of the ovum, 



