V THE DEVELOPMENT OF THE FROG 85 



property required to keep the eggs warmer than the sur- 

 rounding medium. The black pigment of the eggs which 

 readily absorbs the heat rays also functions in the same 

 manner. 



Resistance of the Sexual Products to Cold. — The egg 

 masses of the frog are laid so early in the spring that the 

 water containing them is frequently frozen. Fischer-Sigwart 

 records finding egg masses which were frozen solid for two 

 days, during which the temperature sank to — 8° C. When 

 the eggs were gradually thawed out, they underwent a normal 

 process of development, although somewhat slower than 

 usual, and gave rise to larvae which left the jelly two days 

 afterward. How long eggs may be frozen and how low a 

 temperature they can endure and still retain their power of 

 development is not determined. 



The spermatozoa of the frog may also be frozen without 

 fatal results. The Abbe Spallanzani showed that spermato- 

 zoa frozen in ice for half an hour are able to cause eggs to 

 develop, but if kept in ice for several hours, this power is lost. 

 The spermatozoa apparently have less power of resistance to 

 cold than the eggs ; careful comparisons, however, have not 

 yet been made. 



Structure of the Undivided Egg. — The eggs of the 

 frog as they occur in the body of the female after their 

 discharge from the ovary are surrounded by a very thin vitel- 

 line membrane, which represents its cell wall. On one side, 

 representing the animal pole, the egg is colored by black 

 pigment, which, as is shown in sections, is mainly confined to 

 the periphery immediately under the vitelline membrane. 

 This pigment is in the form of minute granules embedded in 

 the protoplasm. The nucleus of the egg lies excentrically 

 near the middle of the dark pole. When the egg is in the 

 ovary, the nucleus is large and contains a large amount of 



