SECOND PERIOD OF DEVELOPMENT. 63 



tion of Figs. 24, 26, 29, 31, and 33. In the examination 

 of the figures, regard is first paid to the more sharply 

 marked part of the curvature, which is called the an- 

 terior end. By this means I came to the conclusion 

 that the gastrula-mouth belongs entirely to the later 

 dorsal surface, and that the posterior edge of the same 

 marks the posterior end of the embryo. The longi- 

 tudinal axis is constructed accordingly, a straight 

 line being drawn from the sharply marked part of the 

 curvature which marks the anterior end through the 

 posterior edge of the gastrula-mouth. This line crosses 

 at an acute angle the axis drawn from the upper to 

 the lower pole. 



The closing of the gastrula-mouth starts from its 

 anterior edge, while the posterior edge remains all 

 along unaltered. The growing together of the edges 

 follows in a line which forms the larger and posterior 

 part of the later dorsal line. The most posterior re- 

 mainder of the gastrula-mouth still persists for a long 

 time as a small opening dorsally situated at the pos- 

 terior end. 



When we compare the stage which is represented in 

 Figs. 24 and 25 with those represented in Figs. 33 

 and 34, we come at once to the conclusion, that the 



