Ch. II] POLAR BODIES AXD FERTILIZATION 19 



substance around it soon after entering the oviduct, i.e. before 

 it has reached the first part of the convoluted portion. Tliis is 

 the so-called chorion, — a thin investino- membrane which ad- 

 heres closely to the vitelline layer around the egg. During the 

 remainder of the passage through the oviducal tube the egg gets 

 two otlier distinct gelatinous layers (Fig. 10). The middle 

 layer of the three is, according to Newport, a watery layer of 

 considerable thickness. The outer gelatinous covering is also 

 thick and serves to stick the eggs together in a bunch, and even 

 to stick the bunches of eggs, when laid, to surrounding objects. 



The si)awning of certain species of frogs takes place very rap- 

 idly, and by a single effort. Newport says that the process 

 takes place in a few seconds or less than a minute, and that all 

 the eggs that have accumulated in the uteri are laid at once. 

 When laid, the egg-cluster forms a rounded mass which is, at 

 first, scarcely as large as a walnut. The eggs then seem to con- 

 sist almost entirely of dark-colored "yelks" with thin gelatinous 

 envelopes. "Up to about this period the ova remain undisturbed 

 in the water in a mass as they are expelled, and lie indiscrimi- 

 nately, some with the dark and some with the white portion of 

 the yelk uppermost or horizontal. But during the time that 

 has passed since the ova have been in contact with the water, 

 the envelopes have imbibed fluid and expanded until these in- 

 vestments of the yelk have a thickness equal to about two- 

 thirds of the diameter of the yelk itself." 



"The yelks, that have remained up to this time with their 

 white surface uppermost, now change their position spontane- 

 ously by a partial rotation of the whole mass oi each on its 

 axis, within the vitelline membrane, until the dark surface of 

 the whole is placed uppermost. Whether this change of posi- 

 tion is merely the result of expansion of the vitelline membrane 

 at this period, or whether it be also connected, as we may 

 fairly believe, with changes going on in the interior of the 

 yelk, I am not prepared to decide." 



The Jelly of the Egg, and the Second Polar Body 



The jelly around the frog's egg serves, no doubt, as a pro- 

 tection to the egg. The soft eggs are kept in spherical shape 



