82 OUTLINES OF CHORDATE DEVELOPMENT 



face a thick layer, rather firmer than the middle layer. The 

 two thick outer layers may, in some forms, be separated by a 

 narrow dense layer which is distinctly fibrous in structure 

 (Fig. 28). 



The functions of the jelly are various. It serves to some 

 extent to attach the egg masses, but chiefly to protect the 

 eggs from pressure or mechanical injury, from being eaten by 

 other organisms, from infection of various kinds. It seems 

 likely, too, that the jelly assists in the elevation of the temper- 

 ature of the egg, for as a transparent sphere it condenses the 

 heat rays of sunlight which it allows to enter freely and at 

 the same time checks their radiation from the egg. The black 

 pigment of the upper pole seems to function toward the same 

 end by absorbing readily the heat rays, so that altogether the 

 temperature of the egg may be considerably higher than that 

 of the surrounding water. While the eggs, and the spermatozoa 

 also, are very resistant to cold, they are at the same time very 

 sensitive to warmth, so that this slight elevation of temperature 

 has the effect of hastening development an effect that may 

 be quite important since the temperature of the water is often 

 quite low at the time the eggs are laid, and the ponds in which 

 the frogs spawn are quite likely to dry up during the summer, 

 so that each day gained in development toward metamorphosis 

 may mean much as regards survival. 



Maturation. Another effect of the entrance of the sperm is 

 the completion of the maturation process in the egg nucleus. 

 As the sperm enters, this is in the mesophase or metaphase of 

 the second polar division (Fig. 26, /). This division is then 

 rapidly completed and the second polar body cut off; this 

 usually occurs about thirty minutes after entrance of the 

 sperm. The second polar body is of the same size as the first, 

 or smaller. The egg nucleus then reforms in the usual man- 

 ner. The polar bodies are only loosely attached to the sur- 

 face of the egg and frequently may be found floating in the 

 perivitelline space. 



By the time the egg nucleus has reformed the sperm nucleus 

 has also become typical in form, and the two nuclei move 



