i6o Vertebrate Embryology 



on this day, but its development will be de- 

 scribed at a later time. 



The amnion usually makes its appearance 

 during the second day, though, as has been 

 noticed, it may sometimes be seen as a very 

 small rudiment at the end of the first day. 



As was stated in the summary of develop- 

 ment, the head-fold of the amnion is the first 

 to appear. The mesoblast in spreading does 

 not extend into the region immediately in front 

 of the head : this small area, which consists of 

 but two layers, the ectoblast and the entoblast, 

 is known as the pro-amnion (Fig. 46, pr. am), 

 and it is in this place that the head-fold of the 

 amnion is formed ; therefore this head-fold is, 

 for a time, made up only of ectoblast, but at a 

 little later period the mesoblast extends into it, 

 so that it, like the tail- and lateral-folds, is made 

 up of both ectoblast and mesoblast. The actual 

 method of development of the head-fold of the 

 amnion was sufficiently described in speaking 

 of the cloth model (page 114), and attention 

 may be again called to Fig. 38. 



By the end of this day, the head and neck 

 of the embryo are covered by the amnion ; the 

 tail- and lateral-folds of the amnion are well 

 started, but are not so far advanced in develop- 



