228 REPORTS OF INVESTIGATIONS AND PROJECTS. 



that undue reliance had been placed upon the promises of vokmtary observ- 

 ers. As occurs in so many works of similar character, former estimates as 

 to the time of the completion of the monograph must be exceeded, and it now 

 seems probable that it will be impossible to submit manuscript and plates 

 before the spring of 1908. 



Mark, E. L., Harvard University, Cambridge, Massachusetts. Grant No. 

 433. Study of maturation and early stages in the development of the 

 ova of mice. (For previous report see Year Book No. 5, p. 246.) $300. 



For the study of the ova of mice the stock of individuals has been kept most 

 of the time at between 400 and 500. From these mice during the year about 

 225 adult females have been carefully observed and killed at appropriate 

 times; likewise some 50 young females varying in age from young embryos 

 to young animals several weeks old. Somewhat more than half this material 

 has been sectioned and studied. The great amount of time required both 

 for the care and observation of the mice and for the histological preparation 

 of the material has made it impossible for Mr. Long to complete a survey of 

 the whole material, even with the help of an assistant to feed the mice and 

 the employment of a student to do some of the sectioning. 



The thousand or more eggs which we are now engaged in studying form 

 a pretty complete series extending from a period immediately before the 

 beginning of maturation to that of the formation of the two pronuclei ; and 

 yet some of the most important stages are few in number or altogether want- 

 ing, so rapidly do certain parts of the maturation take place. We shall con- 

 centrate our efforts toward obtaining these rarer stages. 



Although there is considerable individual variation, these points have been 

 established : 



(i) Ovulation occurs about 15 hours after parturition. 



(2) Under favorable circumstances the whole process of maturation may 

 be completed in about 8 hours. 



(3) The first maturation spindle is always formed before ovulation. 



(4) The first polar cell is formed before or during the rupture of the 

 follicle. 



(5) The second maturation spindle is formed immediately after the ap- 

 pearance of the first polar cell, and in most cases persists till a sperma- 

 tozoon comes in contact with the egg ; or, if fertilization does not take place, 

 till the egg degenerates. It follows that, normally, the spindle seen in eggs 

 taken from the oviduct is the second maturation spindle, whether the first 

 polar cell is present or not. 



(6) Whether a given spindle is the first or second maturation spindle may 

 be determined by the character of the chromosomes. Those of the first 

 spindle are largre and of characteristic shape and structure ; they are tetrads. 



