EMBRYOLOGY OF CRYPTOBRANCHUS 489 



In making these measurements, it is especially necessary to 

 guard against using eggs with an unusually large yolk plug, 

 since this is one of the commonest abnormalities. Moreover, 

 even in perfectly normal eggs there is considerable variation in 

 the position of the blastopore, so that a large number of eggs 

 must be studied and averages taken. The results obtained by 

 the two methods agree closely. 



Since the blastopore, at the time of its first appearance is- only 

 about 15 degrees below the horizontal equator and approximately 

 parallel to it, the blastopore at first forms an arc of an imaginary 

 circle whose diameter, measured along a meridian of the egg, 

 is about 150 degrees. At the time when the blastopore has 

 reached the form of a semicii cle, this diameter measures about 

 125 degrees; when the blastopore has become a complete circle 

 the average diameter, in normal embryos, is only 94 degrees. 

 Therefore the crescentic blastopore forms an arc of a circle of 

 steadily diminishing diameter; the lips of the blastopore, and 

 the entire germ ring (to be described in a later paper), contract 

 as they progress slowly downward over the egg. 



In preserved material the cleavage pattern of the macromeres 

 is still fairly well defined ; by means of careful study with a binoc- 

 ular microscope it is usually possible to distinguish first and 

 second cleavage furrows (figs. 113 and 114). This enables a 

 direct comparison to be made between the direction of the first 

 cleavage furrow and the median plane of the gastrula; this point 

 will be discussed in a later paper. The identification of early 

 cleavage furrows in this region is furthermore of importance in 

 enabling one to determine the position of the vegetal pole, since 

 this is located at the intersection of the first two cleavage furrows. 

 Measurements show that at the time when the blastopore is first 

 clearly established, the vegetal pole lies, on the average, 68 

 degrees below it, and 7 degrees above the lower pole of the verti- 

 cal axis. At the time when the blastopore becomes a semicircle, 

 the vegetal pole lies only 32 degrees below its dorsal lip; when 

 the blastopore first becomes a complete circle the vegetal pole 

 lies only 26 degrees below the dorsal margin of the yolk plug. 

 During this time continued rotation of the egg has brought its 



JOURNAL OF MORPHOLOGY, VOL. 23, NO. 3 



