The Development of the AlHgator 247 



STAGE IV 



Figures 7a- 7/? (Plates XI., XII.) 



No surface view of this stage was seen by the 

 writer, and hence is not figured. The figures were 

 drawn from one of the series of sections obtained 

 through the courtesy of Prof. S. F. Clarke. This 

 series was marked "3 Urwirbeln, " so that the 

 embryo was apparently slightly younger than the 

 youngest stage obtained by myself and represented 

 in Figures 8 and 8a. 



Figure ya represents a section that passed 

 through the head-fold of the amnion (a) just in 

 front of the head-fold of the embryo ; the amniotic 

 cavity here appears as a large empty space. 



Figure 7& is several sections posterior to the 

 preceding; it passes through the head-fold of the 

 embryo, but is just back of the head-fold of 

 the amnion. The deep depression of the ectoderm 

 (ec) and entoderm (en) caused by the head-fold is 

 plainly seen. In this depression lie the ends of 

 the medullary folds, distinct from each other both 

 dorsally and ventrally. Each medullary fold is 

 made up of two parts — a medial, more dense nerv- 

 ous layer (nl), and a distal, less dense epidermal 

 layer (ep) . The section corresponding to this one 

 will be more fully described in connection with 

 the following stage. 



Figure yc is some distance posterior to the pre- 

 ceding, though the exact distance could not be 



