MAMMALIA. 477 
side by the smaller or epiblast, except at one pole, the 
blastopore. It is difficult to withhold from this stage a 
homology with the gastrula of lower types, such as Amphi- 
oxus. It is sometimes called a “ metagastrula.” The 
embryo has now reached the uterus and then commences a 
remarkable process. The blastopore closes, and the whole 
Fig. 328.—TuREE EARLY Staces IN DEVELOPMENT 
oF Rassir. (After VAN BENEDEN.) 
VITELLINE BLasTorONe 
eumanne 
nN 
N JEPIDAST 
1, The metagastrula; 2, the commencement of the rapid enlargement of 
the egg; 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 d/astocys?, 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 
