72 



J. Thos. Patterson. 



the center of the blastoderm can be made to coincide with that of 

 the micrometer. The numbers at the sides of the ruled area allow 

 one to determine quickly the dimensions of the blastoderm, and at 

 the same time the record of an injury in any quadrant is easily 

 read in the terms of its co-ordinates. 



In operating with this instrument there is a three-fold advantage 



Fig. IV. Microscope-stage incubator used in studying the living egg 



over the free hand method: (1) the place to be injured is easily 

 located; (2) the injury is made with mechanical precision; and 

 (3) consequently the results obtained for any given set of opera- 

 tions are practically constant. 



In connection with the experimental work as well as with the 

 study of sections it is important to make direct observations on the 



