Gastrulation in the Pigeon's Egg. 



Yl 



cells (see Fig. II), is held in the clamp, and by means of the universal 

 joint the point of the needle is brought to bear directly over the 

 blastoderm. To make the injury, the operator observes the magnified 

 blastoderm (magnified 12.6 diameters) through the binocular, and 

 with the right hand moves the needle horizontally by the mechanical- 

 stage until the needle-point is directly above the place to be injured. 

 The point is then inserted- by adjusting the rack and pinion with the 

 left hand, and the circuit is completed immediately by touching 

 the second needle to the albumin. The extent of the injury can 

 be regulated either by the number of battery cells included in the 

 circuit, or by the length of time the current is allowed to run. 







4 3 2 10 1 2 3 4 



Fig. III. Eye-piece micrometer whicli is placed in one of the oculars of 

 the binocular, and thus the blastoderm appears to the observer as plotted 

 into small squares. 



In order not to expose the blastoderm unduly while operating on 

 early stages, it is highly desirable to have some quick and easy method 

 for locating the place to be injured. This is done by using a net- 

 micrometer, which is placed in one of the oculars, and thus the blasto- 

 derm is plotted into small squares. Two grades of micrometers are 

 used, one ruled into 0.1 mm. and the other 0.5 mm. squares. A draw- 

 ing of the latter kind is shown in Fig. III. In practice, the egg is 

 placed in a depression at the top of a large cork, which, with the egg, 

 can then be moved about on the stage of the binocular. In this way 



