70 OUTLINES OF CHORDATE DEVELOPMENT 



variable sizes. The egg of the frog is therefore markedly 

 tclolccithal. 



In the animal pole the nucleus is contained. At the time of 

 egg laying this is in the metaphase of the second polar division 

 (Fig. 26, /). The first polar body has already been extruded 

 and though very small, may be found near the light spot in 

 the flattened area at the upper pole. 



X__ .,nrmrr-iiin i.iuiiii J..L.J-. ' " * 



FIG. 23. Egg and spermatozoa of frog. A. Spermatozoon of Rana fusca. 

 After Broman. B. Spermatozoon of R. esculenta. After Broman. C. Section 

 through the fully formed ovarian egg of Rana sp. From Morgan (Development 

 of the Frog's Egg). The protoplasmic animal pole is covered with a thin layer 

 of pigment; vegetal pole filled with yolk bodies; other deutoplasmic granules are 

 distributed throughout the cell. The large nucleus, or germinal vesicle, sur- 

 rounded by a definite nuclear membrane, lies eccentrically toward the animal 

 pole, and contains the thread-like chromosomes and a group of nucleoli. 



The specific- gravity of the deutoplasm is slightly greater than 

 that of the protoplasm, and this brings about the assumption 

 of the definite position of the egg with the vegetal pole down- 

 ward. But at this time the egg membranes are so closely 

 adherent that its rotation to this position may be very slow. 



At this time, i.e., preceding fertilization, the only symmetry 

 of the_egg is that expressed by its polarity. That is to say it 

 has no single plane of symmetry, only an axis of symmetry 

 (polar axis); this is the primary egg axis passing through the 

 middles of the light and dark (vegetal and animal) poles. 



