MAMMALIA. 477 



ide by the smaller or epiblast, except at one pole, the 

 )lastopore. It is difficult to withhold from this stage a 

 lomology with the gastrula of lower types, such as Amphi- 

 xus. It is sometimes called a " metagastrula." . The 

 imbryo has now reached the uterus and then commences a 

 emarkable process. The blastopore closes, and the whole 



Fig. 328. — Three Early Stages in Development 

 OF Rabbit. (After Van Beneden.) 



I, The metagastrula ; 2, the commencement of the rapid enlargement of 

 the Qg^ ; 3, the fully-formed blastocyst. 



embryo increases rapidly in size. The epiblast-cells become 

 flat and continue to divide, keeping pace with the great 

 increase in size, but the hypoblast- cells remain in a small 

 heap at the blastoporic pole. Thus is produced the so- 

 called blastocyst, its large cavity filled with a colourless fluid. 

 The hypoblast-cells may now increase and commence to 

 spread round the inner surface of the vesicle, or, as in the 

 hedgehog, they may split to form an internal cavity or sac 

 and then expand. In either case the same result is attained 



