EMBRYOLOGY OF TADPOLE AND CHICK 551 



(which also has only half of the normal number of chromosomes) now 

 enters the egg and leaves a trail of pigment behind it. This sperm, after 

 entry into the egg, becomes the male pro-nucleus. The head of the sperm 

 makes its way directly to the female pro-nucleus. The tail of the sperm 

 is thrown off although both tail and midpiece enter the penetration path 

 (Fig. 319). The head and midpiece after traveling for some distance in 

 the egg, rotate so that the midpiece is placed in advance of the head. 

 The midpiece then begins to dissolve and to form a typical nucleus with 

 an opening within which is called a vesicle. The sperm then changes its 

 course, and moves toward the point where male and female pro-nuclei 

 will unite, unless, of course, the penetration path has already led in that 

 direction. The path made by the changing of direction of the head of the 

 sperm is called the copulation path (Fig. 319). This path is also marked 

 by a trail of pigment as the head of the sperm passes through the cyto- 

 plasm to reach the female pro-nucleus. 



Fig. 319. 



Sections through the egg of R. fusca, showing penetration and copulation 

 paths, and the symmetry of the first cleavage plane. A. Sagittal section through 

 the egg before the appearance of the first cleavage ; B. Frontal section of the 

 same stage as A, showing the symmetrical distribution of the egg material. C. 

 Frontal section through egg in two-cell stage, showing the symmetry of the egg ; 

 the penetration path is not shown, a. Anterior ; cp, copulation path ; I, left ; p, 

 posterior ; pp, penetration path ; r, right ; , remains of first cleavage spindle ; 

 sp, superficial pigment; 1, first cleavage furrow. (After O. Schultze.) 



After the sperm has entered the egg, some of the fluid from the egg 

 proper is withdrawn into a space between the egg itself and the chorion. 

 This is known as a perivitelline space. The egg can thus rotate within 

 its membrane. From this time onward the pigmented pole is always 

 uppermost. In unfertilized eggs, the membranes are more or less ad- 

 herent. The jelly-like covering of the egg absorbs considerable fluid and 

 swells up in about a minute after the egg touches water. A close ob- 

 servation of the jelly shows that it is made up of various layers, whose 

 function is not only to protect the egg from chemical and mechanical 

 injuries, and from being eaten by other organisms, but also to elevate the 

 temperature of the egg. They accomplish this latter by being trans- 

 parent spheres which condense the heat rays from the sun and at the 

 same time check the radiation from the egg itself. 



