2 THE MATURATION OF THE EGG OF THE MOUSE. 



Since the summer of 1906 papers on this subject have been published 

 by Gerlach, Coe and Kirkham, Kirkham, Lams et Doorme, and lastly 

 by Sobotta. It is a satisfaction to confirm some of the results of these 

 investigators. There are, however, a number of points in which we do 

 not agree with any of our predecessors ; some of these are due to differ- 

 ences of interpretation, some to differences of technique, and others to 

 the insufficiency of material at the command of some of those who have 

 preceded us. 



A considerable part of the expense incurred in maintaining and 

 caring for the mice has been covered by a grant from the Carnegie 

 Institution of Washington, and a part of the same grant has been used 

 in procuring the assistance of an aid to do part of the less important 

 technical portion of the preparation of slides. 



II. LITERATURE. 



It is not our intention to give here a summary of the subject of 

 the maturation of the egg of either invertebrates or vertebrates. The 

 reader is referred to Boveri (1892), Rlickert (1894), Hacker (1899), 

 Korschelt und Heider (1903), and Gregoire (1905) for excellent general 

 reviews of the literature of the whole field or special portions of it; to 

 R. Hertwig (1903) for similar information relative to vertebrates; and 

 to Sobotta (1895) an d Kirkham (19076) for surveys of the papers on 

 mammals. The following brief account of the several works on the 

 mouse will serve as an introduction to the results set forth in this paper. 

 More detailed references will be made wherever necessary. 



The first to study the egg of the mouse was Bellonci (1885). He 

 described in ovarian eggs the spindle and the chromosomes arranged at 

 its equator and considered them as being similar to those of some inver- 

 tebrates. According to his account the first polar cell and the second 

 spindle are formed while the egg is still in the ovary. The polar cell he 

 considered a true cell with a membrane. 



Tafani (1889) studied both living and preserved eggs. He believed 

 that the chromosomes of the first spindle, numbering twenty, were 

 formed from the nucleolus while the egg was in the ovary, but that the 

 division of the first spindle and the formation of the first polar cell took 

 place after ovulation. He thought that in one-fifth of all cases the 

 chromosomes left in the egg after the formation of the first polar cell 

 produced a second spindle, while in the remaining four-fifths they were 

 directly transformed into the female pronucleus. Thus, in his opinion, 

 in about one-fifth of the eggs two polar cells were produced, while in 

 four-fifths there was only one, the second polar cell being in the latter 

 suppressed. No explanation of the cause of this difference was offered. 

 He said that each of the polar cells contained either a nucleus or granules, 



