THE MATURATION OF THE EGG OF THE MOUSE. 



By J. A. Long and E. L. Mark. 



I. INTRODUCTION. 



Researches into the maturation phenomena of both plants and 

 animals have been extended greatly in recent years, and, although they 

 have given rise to numerous different and sometimes conflicting theories, 

 they point on the whole toward a striking uniformity of processes for 

 all of the forms of life studied. Among the metazoa investigations 

 have covered not only the maturation of eggs, but also the production 

 of spermatozoa. These investigations have shown the general rule to 

 be that by means of two mitoses, not separated from each other by a 

 resting nuclear stage, there are formed in the one sex a ripe egg and two 

 (or three) polar cells and in the other sex four spermatids. In many 

 cases the origin, structure, and divisions of the chromosomes involved 

 in these mitoses have received particular attention. 



The greater number of works on the maturation divisions of eggs 

 have been carried out on invertebrates, which furnish the most easily 

 obtainable material. Work on vertebrates has been largely devoted to 

 the study of amphibians and mammals. In the case of mammals, which 

 perhaps present the most interesting field for the study of oogenesis, 

 the investigation is especially difficult, since the kinds of mammals 

 lending themselves to such researches are for several reasons relatively 

 few; among these reasons are the large size of the more common domestic 

 forms, the difficulty of breeding wild animals in captivity, and the 

 infrequency of the breeding periods. Of the mammals most carefully 

 studied (bat, rabbit, guinea-pig, and mouse) the last has been believed 

 to be the only exception to the general rule that two polar cells are formed 

 in the maturation of the egg. 



According to the excellent works of Tafani and Sobotta, the egg 

 of the mouse forms two polar cells in only a small proportion of cases; 

 in the greater proportion of instances it produces only one polar cell. It 

 was because of this apparent exception to the general law of maturation 

 in metazoan eggs that the present piece of work was undertaken. It was 

 begun in 1903 with the hope of finding some explanation for the sup- 

 posed two classes of eggs. 



It soon became clear that it would be necessary to go over the whole 

 subject in a systematic way on the basis of the changes taking place in 

 the chromosomes. To do this thoroughly has involved so much time 

 that it has not been possible to give special attention to the cytoplasm. 



