134 Gastrulation and Embryo Formation in Amia Calva 
ous assumption that the egg of Amia is meroblastic. In view of these 
facts, the present writers have thought that a renewed study of these 
phases of development might be profitable. 
The ages given in the following description of stages have been deter- 
mined from material taken from a single nest. The eggs were taken from 
the nest and placed in dishes which were submerged in the lake, a con- 
stant temperature of about 16° C. being thus maintained. It is well 
known that no two spawnings progress at precisely the same rate. It is 
thus obvious that the ages designated are only in a general way indicative 
of the degree of development. We have, therefore, given measurements 
of the extent of the blastodise and embryo in addition to the age. 
The description of the latest stage studied by Whitman and Eycleshy- 
mer reads as follows: “ The calotte, which has now begun to extend over 
the yolk, consists of thickly crowded spherical cells which marginally 
pass abruptly into the large yolk segments, while in the central portion 
they gradually increase in size and lie loosely scattered. The outer layer 
of the calotte is distinctly differentiated in that the cells are elongated 
and more densely granular. The entire yolk is irregularly cleft, the cells 
forming the lower portion are roughly polygonal and grade off into the 
larger yolk spheres which lie at the center.” This stage of development 
indicates the beginning of our study. 
DETAILED DESCRIPTION OF STAGES. 
Egg Nine Hours After Fertilization. Blastodisc Covers About 100° 
of the Circumference of the Egg.—A profile view of an egg of this age 
is shown in Fig. 1. An examination of the surface of the blastodise 
shows an area at the upper pole of the egg in which cell division is most 
rapid. In addition to this, there are frequently found other areas in 
which cell division is accelerated. Often one side of the blastodise is 
distinctly in advance of the other. Again, the most careful search results 
in a failure to detect such areas. We, therefore, are unable to say what 
relation, if any, these areas bear to the future embryo. 
In all eggs of this stage the surface of the yolk is cleft by thirty to 
forty furrows which pass in meridional planes. Many of these grooves 
have not as yet reached the vegetative pole. Some never reach the pole, 
but pass obliquely into the longer ones. Through this process a number 
of long triangular segments are cut off at the upper margin of the yolk, 
as shown in the figure. At the lower pole, where fifteen to twenty grooves 
converge, the yolk is irregularly cleft. In general it may be said that the 
cleavage of the yolk as compared with the cleavage of the blastodisc is 
exceedingly slow. 
