W. G. YanName — Embryology of Eustylochus. 287 



appearing ; in othei- preparations they become very pale and appear 

 to dissolve away in the contents of the pronucleus. 



These nucleoli seem to be waste matter, and resemble in every 

 particular the nucleoli of the male pronucleus, and those of the 

 nuclei of the cleavage stages. 



While these changes have been going on, the female pronucleus 

 has moved a little toward the center of the Qgg, keeping, 

 however, on or near the line from the center to the animal pole. 

 The aster of the second polar spindle, which remains in the Qgg, 

 meanwhile degenerates, the centrosome and centrosphere disappear, 

 and all that remains is a spherical mass of bluish stainy granules, 

 apparently the microsomes of which the aster rays were made up 

 (Figs. 19a, 20 and 21). This also moves to a more central position 

 in the egg. In the eggs of Planocera it is much less conspicuous 

 than in those of Eustylochus, where, however, it is much more 

 noticeable in some preparations then in others. No yolk globules 

 are found within the area occupied by this granular mass. 



The Male Pronucleus. 



■I have given an account of the spermatozoon up to the time when 

 the egg reaches the uterus. No change appears to take place in it 

 until after the egg is laid. In a recently laid egg (Fig. V) the dark 

 colored area in the egg cytoplasm by which one end (probably the 

 posterior, or middle piece) of the spermatozoon was surrounded, has 

 disappeared and the spermatozoon has contracted into a shorter and 

 thicker form, so that it is now spindle shaped. During the succeeding 

 stages of the first polar spindle it contracts still more, becoming 

 irregularly rounded or oval. It also loses its homogeneous appear- 

 ance and becomes transformed into a vesicle in which a reticulum, 

 and later one or more nucleoli similar to those in the female pro- 

 nucleus, make their appearance (Figs. 18, 39 and 40). 



It is generally still quite small at the end of the maturation 

 period. In its final stages it appears to have precisely the same 

 structure as the female pronucleus, but may generally be dis- 

 tinguished by one or more of the following characteristics : its 

 greater distance from the animal pole, more regular form, slightly 

 smaller size, and smaller number of nucleoli. As a comparison of 

 the above figures will readily show, its development does not 

 necessarily keep pace with that of the female elements, but each 

 develops independently of the other. 



