60 THE MATURATION OF THE EGG OF THE MOUSE. 



3. Second spindle. 



Chromatin. 



It was Tafani (1889, p. 23) who first announced that in the greater 

 number of cases in mice only a single polar cell is formed. It was 

 therefore his opinion that the chromosomes which remained in the egg 

 after the formation of the first polar cell gave rise either to the second 

 spindle (few cases) or to the female pronucleus (greater number of cases) . 

 This opinion would be the natural consequence of his probable confusion 

 of the second spindle with the first. Sobotta in his early paper (1895, 

 p. 44) also held that in those eggs which produced but one polar cell 

 (in nine-tenths of the cases, in his opinion) the spindle was formed 

 directly from the germinative vesicle, and (1895, p. 53) that in all other 

 eggs (one-tenth of the total number) the second spindle was produced 

 from the chromosomes which remained in the ovum after the first polar 

 cell was abstricted. Since Sobotta considered the spindle in the former 

 instance to be the equivalent of that in the latter, it follows that, accord- 

 ing to his view, the second spindle was formed in some cases directly 

 from the germinative vesicle. In a later paper (1907, p. 514) he says 

 that he has no observations to prove this view and that it is erroneous. 

 As stated in this paper (1907, p. 519), he now believes that (in a larger 

 proportion, about one-fifth of the cases) the second spindle originates 

 as previously described for one-tenth; but in 4 out of every 5 eggs the 

 monaster of the second spindle is derived directly from the monaster of 

 the first, i.e., without the formation of a polar cell. That is, the first 

 spindle in a large proportion of ova does not divide, but, in some way 

 which involves a degeneration of half of the chromosomes within the 

 cytoplasm of the egg (1907, p. 541; 1908, p. 250), is transformed into 

 the corresponding condition of the second spindle. This belief he thinks 

 accords with his observation that in preserved material the occurrence 

 of the division of the first spindle is very infrequent. 



This is Sobotta's explanation of the occurrence of only one polar 

 cell in many oviducal eggs in the late stages (the ones he worked with 

 chiefly. See pp. 14, 45). It is not based on any observation of degen- 

 erating chromosomes or of the supposed stages of transformation. In 

 fact, Sobotta repeatedly says that he has seen no such stage, although he 

 believes that in a single instance (1907, fig. 8, a spindle with more than 

 16 chromosomes, which occurred in an oviducal egg) he may have had 

 an example. It should be noted that, if this transformation occurs in 

 four-fifths of all the eggs, the chances of meeting with it must be four 

 times as many as the chances of encountering the division of the first 

 spindle. In view of these considerations one may be warranted in ques- 

 tioning the existence of such a condition. 



Gerlach (1906, fig. 6) illustrates an early stage in the origin of the 

 second spindle , with which the description of the same stage in the pres- 

 ent paper agrees. 



