138 



HISTORY OF THE GERMINAL LAYERS. 



Au inspection of these two tables sliews that an actual diminution in 

 the length of the primitive streak takes place just about the time when the 

 first somites are being formed, but there is no ground for thinking that 

 the primitive streak becomes then converted into the medullaiy plate. 

 Subsequently the primitive streak does not for a considerable time become 

 markedly shorter, and certainly its curtailment is not really sufficient to 

 account for the increased length of the embryo— an increase in length, 

 which (with the exception of the head) takes place entirely by additions at 

 the hind end. At the stage with fourteen somites the primitive streak is 

 still pretty long. In the later stages, as is clearly demonstrated by the 

 tables, the diminution in the length of the primitive streak mainly concerns 

 the posterior part and not that adjoining the embryo. 



General history of the germinal layers. 



The epiblast. Tiie epiblast of the body of the embryo, though 

 several rows of cells deep, does not become divided into two strata 

 till late in embryonic life ; so that the organs of sense formed from 

 the epiblast, which are the same as in the types already described, 

 are not specially formed from an inner nervous stratum. The 

 medullary canal is closed in the same manner as in Elasraobranchii, 

 the Frog, etc., by the simple conversion of an open groove into a 

 closed canal. The closure commences first of all in the region of the 

 mid-brain, and extends rapidly backwards and more slowly forwards. 

 It is completed in the Fowl by about the time that twelve meso- 

 blastic somites are formed. 



The mesoblast. The general changes of this layer do not exhibit 

 any features of special interest — the division into lateral and verte- 

 bral plates, etc , being nearly the same as in the lower forms. 



The hypoblast. The closure of the alimentary canal is entirely 

 effected by a process of tucking in or folding off of the embryo from the 



