Cii. V] EARLY DEVELOPMENT OF THE EMBRYO 55 



long as do those i^rocluced by the first method, but as less harm 

 is done to the egg, one can often get more satisfactory results.^ 



If wlien the first trace of the blastopore appears on the sur- 

 face of the egg (Fig. 19, A), a slight injury is made to the 

 surface of the white hemisphere at the side opposite the blasto- 

 poric lip, i.e. at a point 150 degrees from the dorsal lip of the 

 blastopore (Fig. 19, A, at x), we shall find that in the course 

 of four hours the blastopore will form a crescent, and that the 

 distance from the dorsal lip to the point of injury is much less 

 than at first (Fig. 19, III). A circular line of pigment in the 

 white hemisphere shows tlie line along which the lateral and 

 posterior lips of the blastopore will appear. It will be seen in 

 this case, that the point of injury lies therefore outside of the 

 yolk-plug, i.e. posterior to the ventral blastoporic lip. 



In the course of four liours more it will be found that the 

 circular blastopore is much smaller than before, and that the 

 dorsal lip now lies much nearer to the point of injury (Fig. 19, 

 IV). The dorsal lip has travelled over more than two-thirds of 

 the original distance from its starting-point to the point of 

 injury. By making a new experiment with an egg that has 

 reached this stage of development, it will be found that when 

 the outlines of the blastopore have become sharply defined, 

 the later closure takes place at nearly an equal rate from all 

 points of the circumference, perhaps, however, still somewhat 

 more rapidly from the dorsal lip backwards. 



Where the diameter of tlie circle representing the outline of 

 the egg equals 27 mm.,^ the distance between the blastopore 

 and the injury measures 21 mm. In the first four hours the 

 blastopore moves through 8 mm. In the next four hours it 

 travels through 7 mm., and is therefore now only 9 mm. from 

 the point of injury. By this time the blastopore is circular in 

 outline, and the injury lies just outside (2 mm.) of the circle. 

 The blastopore now measures 7 mm. in diameter. Assuming 

 that from this time forward the blastopore grows together at an 

 equal rate toward its centre, then the dorsal lip will pass over 

 about one-half of the diameter of the blastopore, or 4 mm. 



1 This method can be used only with great caution. 



^ The numbers refer to the measurement of the figure, and not to the egg itself. 



