Noi2.) ZHE EGG OF AMIA AND ITS CLEAVAGE. 331 
little thicker on one side than elsewhere. The germinal ves- 
icle lies eccentrically beneath the deeper side, mainly in the 
yolk, only its upper surface projecting into the calotte. There 
are thus indications of a definite orientation in the egg before, 
as well as immediately after, fertilization. 
First Cleavage. — In the egg shown in Fig. 3 the first cleav- 
age appeared at 7.54 A.M. (2 hrs. 24 min. after deposition). A 
slight flattening of the animal pole precedes the appearance of 
this groove, which generally divides the calotte into nearly 
equal portions. The groove travels more and more slowly as it 
passes beyond the margin of the calotte. Its decreasing rate of 
progress in the egg figured is indicated by the following per- 
centages, based upon the length of the are traversed at succes- 
sive intervals of about 1 hr. each, as compared with the entire 
circumference. At 8.53 (Fig. 3) the arc described equals 36% 
of the entire circumference ; at 9.49 (Fig. 4) 60%; at 10.50 
(Fig. 5) 77% ; at 11.55 (Fig. 6) 89% ; between 11.55 and 12.58 
(Fig. 7) 100%. In most cases this cleavage is not complete 
until after the appearance of the fourth and sometimes even 
the fifth set of grooves. 
The first groove is most frequently meridional, as shown in 
Figs. 3, 21, 22, and 23. In some cases, however, it may depart 
so far from a meridional that the segments formed are quite 
unequal (cf. Dean, /c., p. 425), as shown in Fig. 9. The ends 
of the groove may meet at the lower pole so as to form a con- 
tinuous straight line, or in such a manner that an obtuse angle 
is formed (Figs. 14, 17). Sometimes they do not meet at all, 
and are only united by a portion of the second, or even third, 
cleavage grooves. 
At the time the second groove appears the first has a depth 
of about one-half the thickness of the calotte, as shown in Cut 
2 (made in plane of dotted line of Diagr. A). The path of 
this groove is definitely premarked to a point about twice this 
depth, terminating in an irregular vacuolar space at the level of 
the nuclei. The depth of the cleavage at this time strongly 
reminds one of the conditions seen in teleostean eggs with a 
perfectly defined blastodisc. The calotte certainly represents 
a concentration of germinal material, analogous to the blasto- 
