CHROMOSOMES OF HYDATINA SENTA. 57 



to collect in masses, so that the number of specimens clear enough 

 to furnish counts was not large. 



GENERAL COURSE OF MATURATION. 



Inasmuch as I was primarily interested in the number of 

 chromosomes, in each of the kinds of females, I have made no 

 attempt to render a complete account of maturation. Such 

 facts as have been revealed, even though incidental to the main 

 object, are here recorded. Only that part of the maturation 

 which occurs in the oviduct of the female has been studied. The 

 completion of the process after the egg is laid has not been fol- 

 lowed. 



About the time when the oocyte has reached its maximum 

 size and is fairly fixed in form (ellipsoidal), not yielding to the 

 movements of the body of the female, the nucleus rapidly in- 

 creases in volume. An aster appears in the cytoplasm near it, 

 being readily visible by the practical absence of yolk spherules, 

 although radiations are very indistinct. So plain is the aster 

 that it was invariably used as a guide-post in locating maturation 

 spindles. The spindle is formed within the nucleus. Upon it 

 the chromosomes appear as long slender threads tapering toward 

 both ends (Fig. i). As the spindle develops the nuclear mem- 

 brane disappears, so that the spindle is out in the cytoplasm: 

 but there is always a definite space in the cytoplasm in which 

 the spindle is located so that there is no confusion of chromo- 

 somes and yolk spherules. With very few exceptions, the 

 spindle, which is near the periphery of the cell, is turned toward 

 the intestine. Exceptions to this rule are more common in 

 male-producers than in female-producers. 



In the female-producing egg the chromosomes arrange them- 

 selves on the equator. In this condition the cell appears to 

 remain until the egg is laid, for out of a large number of specimens 

 not one was found that had proceeded beyond that stage. 

 Lenssen ('98), indeed, concluded that the division was never 

 completed; but Whitney ('09) observed the single polar body 

 at the periphery of the segmenting egg. 



In the male-producing egg several specimens indicate that the 

 chromosomes meet in pairs on the equator of the spindle. The 



