DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 431 
or whether all portions of it, when they are first formed, are 
continuous or separate. 
There can be little doubt that the intra-embryonic ccelom of 
the higher Vertebrata corresponds closely with the ccelomic 
cavity of the lower Vertebrates. The differences which exist 
are secondary ; they are due partly to altered circumstances of 
existence in the later periods of development, and partly to 
the formation of a yolk hernia which has interfered with and 
modified the development of the cavity in the earlier stages ; 
but we are not in a position to say how these modifications 
are produced, for as yet we scarcely understand what they are, 
and how they appear in the ontogeny of any one animal. 
Van Beneden’s and Julin’s researches upon the formation of 
the amnion in the rabbit and bat (8) have added greatly to 
our knowledge of the extension of the extra-embryonic por- 
tion of the body-cavity, but the accounts of this process in other 
mammals differ considerably from that of van Beneden and 
Julin, 
In the sheep the celom arises in the extra-embryonic area, 
as a series of small irregular spaces in the mesoblast. These 
spaces soon fuse, and a continuous cavity results. The forma- 
tion of the celom commences at the anterior and posterior 
ends of the embryonic area, and afterwards extends forwards 
and backwards (6). 
In the cat also, according to Fleischmann (12), the forma- 
tion of the ceelom commences in the extra-embryonic area, 
whence it afterwards extends into the embryonic region, but 
instead of appearing as a series of irregular independent 
spaces, it is from the first a tubular canal; possibly, however, 
Fleischmann has missed the earlier stages, for, according to 
Bonnet’s account, the tubular ccelom succeeds the irregular 
spaces and results from their fusion. 
In the guinea-pig (45) the ccelom appears in the extra- 
embryonic area before the formation of the mesoblast com- 
mences, and it is bounded at first only by epiblast, trophoblast, 
and hypoblast ; its mesoblastic walls are formed afterwards by 
migratory cells, which advance over its primitive walls from 
