488 BERTRAM G. SMITH 



perfect circle, and enclosing a broad horseshoe-shaped band of 

 transitional cells, within which lie the macromeres (fig. 138). 



It has already been noted that a slight rotation of the egg on 

 a horizontal axis has taken place in Stage 10, so that it is now 

 necessary to distinguish between the morphological axis of the 

 egg and the vertical axis determined by gravity, since the two 

 no longer coincide. In the study of gastrulation this rotation 

 must be taken into account, and some means must be found for 

 measuring it. In studying the morphological features of the 

 egg (position of blastopore, etc.) in their relation to the vertical 

 axis, two general methods have been used: (a) the living egg, 

 placed in a small vial of water, has been studied in side view and 

 measurements made against a protractor used as a background; 

 and (b) for accurately locating the vertical axis I have devised 

 the following apparatus : a glass disc such as is used for an ocular 

 micrometer was marked in the center with a small dot; a circle 

 with a radius of 4 mm. was then drawn about this dot as a center. 

 When this disc is placed in the eyepiece of a low-power micro- 

 scope used in studying the eggs, the circle is just large enough 

 to enclose the image of an egg. When the egg, immersed in water 

 in a watch glass, is accurately placed so that its image is enclosed 

 by the circle, the dot lies over the upper pole of the vertical axis ; 

 this point is then marked by puncturing with a hot needle. The 

 operation was first tried on living eggs, which were then fixed 

 for further study; but since with the living egg even a small 

 puncture in this region usually causes the embryo to collapse 

 during the subsequent process of fixation, in general this method 

 is less satisfactory with living than with preserved material. 

 On account of the usually perfect preservation of the form of 

 the egg by the fixing fluid employed, the results obtained by 

 marking preserved material seem fairly trustworthy, especially 

 when spherical eggs are selected and a large number used. The 

 position of the upper vertical pole, thus marked, gives a reference 

 point for correlating the morphological features of the egg with 

 the vertical axis; the measurements were made by means of 

 camera drawings. 



