OBSERVATIONS ON THE MATURATION PROCESSES. 35 



B. OOCYTE II. 

 l. General Description of Stages. 



The chief criterion according to which an egg may be judged to 

 be an oocyte of the first order or of the second order is the character of 

 the chromatin contents. As the sequel will show, this is the only relia- 

 ble standard. It will naturally occur to the reader that the oocyte of 

 the second order must be accompanied by the first polar cell, and that 

 this fact would be a satisfactory criterion. But the following facts com- 

 plicate the situation: first, some fertilized eggs exhibit two polar cells, 

 some but a single one; secondly, there is dispute as to whether this 

 single polar cell is homologous with the first or second one of eggs hav- 

 ing two. In the description of the following stages it will be assumed 

 that the egg naturally has two polar cells, and the question as to how 

 many polar cells are actually formed will be treated of in a later chapter. 



Stage VII. Formation of Second Maturation Spindle. 



(Plate 4, fig. 19.) 



It is fair to infer from the comparatively long duration of the pre- 

 ceding Stage (VI) that the disk-shaped mass of chromatin which re- 

 sults from the more or less complete fusion of the chromosomes left 

 in the egg after the formation of the first polar cell probably remains 

 for some time without perceptible change of morphological conditions, 

 and that the persisting half of the interzonal filaments likewise under- 

 goes little change during this period. With the close of this period of 

 apparent inactivity Stage VII begins. It embraces only the metamor- 

 phosis of the chromatin mass and what are probably the achromatic 

 remnants of the first spindle into the fully formed second maturation 

 spindle. This process, unlike the one involved in the completion of 

 the first spindle, is so rapid that it can not be subdivided into stages 

 and traced step by step. 



Stage VIII. "Equatorial Plate" of Second Maturation Spindle. 



(Plates 4, 5, figs. 20 to 27.) 



As this stage is unique, in that it depends on the occurrence of 

 semination for its normal termination, it may have a greater length 

 than any other part of the whole maturation process, and is therefore 

 the one most easily obtained. If semination is early, the spindle divides 

 without undergoing any previous alterations; on the other hand, if the 

 access of spermatozoa be hindered, the spindle, though remaining com- 

 paratively inactive, undergoes certain changes as a result. 



When newly formed, the second maturation spindle (plates 4 and 

 5, figs. 22 to 24) is very similar to the first spindle immediately before 

 its metakinesis, differing from it only in being a little smaller, in the 

 structure of its chromosomes, and in their more exact arrangement in 

 the plane of the equator. If semination is prevented, the resulting pro- 

 longed quiescence of the spindle is characterized by a diminution in 



