BREEDING HABITS OF ALBINO RAT 305 



when it enters the egg, a fact mentioned by Coe, and by Sobotta 

 and Burckhard. As soon as the sperm head begins to penetrate 

 the cytoplasm of the egg the formation of the second polar body 

 is started. 



In the albino rat the second polar body is characterized by 

 having the chromatin content massed, while the chromatin of 

 the first polar body is always scattered through the cytoplasm. 

 This distinction, however, does not hold for the Norwegian rat, 

 of which two eggs are shown in figures 17 and 18. The chro- 

 matin left in the egg after the formation of the second polar 

 body rounds itself up and becomes surrounded by a membrane, 

 thus forming the female pronucleus. The sperm head on its 

 entrance swells up and likewise assumes a rounded form with 

 a nuclear membrane, as is shown in figure 16. 



SUMMARY 



1. Male albino rats rarely, if ever, are responsible for the kill- 

 ing or maiming of their young. Diseased condition or fright 

 are probably the chief causes of the destruction or injury of their 

 offspring by the females. 



2. Albino rats give birth to young the year round, but only 

 from April to October do the females regularly ovulate twenty 

 to forty-eight hours after parturition. 



3. Albino rats of both sexes are sexually mature when less than 

 two months old. 



4. Living rat eggs are easily obtainable during the four days 

 following ovulation by dissection of the Fallopian tubes. 



5.. The maturing eggs in the ovary are joined to the surround- 

 ing follicle cells by very definite cell bridges. 



6. The development of eggs can be traced in the ovary through 

 two oestrus cycles preceding their discharge. 



7. The first polar spindle is short and broad, and is usually 

 formed less than twenty-four hours after parturition. 



8. The first polar body is always formed in rat eggs, but its 

 protoplasm is very unstable, and disintegrative processes often 

 bring about its complete disappearance about the time the egg 

 reaches the Fallopian tube. 



