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HISTORY OF THE GERMINAL LAYERS. 



through the primitive streak. Where the primitive streak becomes divided 

 into two parts the sections through the two parts are given separately : the 

 left column (A) referring to the anterior part of the streak ; the right 

 column (P) to the posterior part. The third column gives the number of 

 sections through the embryo. The first table is for fowl embryos, the 

 second for goose embryos. 



An inspection of these two tables shews 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 medullary 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 tJie germinal layers. 



The epiblast. The 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 



