
758 The American Naturalist. [August, 
of the blastopore, the so-called overgrowth or epibolic growth of the 
black cells has been observed. I am quite sure, however (except in the 
immediate region on the dorsal side of the blastopore, and later over 
its whole extent), that the yellow cells disappear from the surface not, 
by an overgrowth of the first-formed black cells, dut dy a process of 
splitting off of cells from the upper corner of the yellow cells themselves. 
In other words, there is not a general migration of black cells, but each 
remains approximately in the position in which it was first formed, 
and new black cells are continually added at the periphery of the 
black cap by the splitting off of cells from the upper ends of the yel- 
low cells, so that Balfour’s sentence, that the disappearance of the 
yellow cells ‘‘ is effected by the epiblast growing over the yolk at all 
points of its circumference,’’ is somewhat misleading. Asa corollary 
to what I have said, it follows, of course, that there is a continuous 
formation of new pigment taking place at the periphery of the black 
area within the new cells that are being formed, and also within the 
ends of the yellow cells which go to form the new cells in this region. 
I have not studied with sufficient care the gradual turning in of the 
cells around the rim of the blastopore. In one living egg, however, I 
saw in the dorsal region of the blastopore some of the cells forming 
the floor of the archenteron gradually disappear wiżAin the blastopore. 
7. Effect of Temperature.—lIt is impossible to give any exact time to 
the different stages of development, as the time is directly proportional 
to the temperature of the water within certain limits. The highest 
temperature is not always the optimum, for several bunches placed in 
an incubator for hen’s eggs were entirely destroyed. The freezing of 
the water in which the eggs are kept does not seem to injure the eggs 
in the least, but simply to retard their development. I have had eggs 
completely surrounded by ice, and afterward development quite 
normally. However, when the eggs themselves are actually frozen 
they seem to be destroyed, perhaps by the formation of ice spicules 
within them. The wood frogs, which lay their eggs so early, generally 
lose in this locality great numbers of them on account of getting caught 
in the ice. Those which are not so caught develop later, when the ice 
melts, and do not seem, in any way, to be injured by water at the 
freezing-point. I think there is here a most interesting field for 
experimentation by the physiological embryologist, and I regret I have 
not kept exact records of the effects of heat and cold. 
8. Times of Hatching, etc.—The different species of frogs leave the 
jelly membrane at different ages. Some have the tail well developed, 
and are quite active. Others have the tail just appearing, and are 



