ON THE DEVELOPMENT OF LINGULA ANATINA. 13 



That a micropyle is present piercing the vitelline membrane 

 is almost certain, although it has not as yet actually been deter- 

 mined ; for it will be remembered that the vitelline membrane is 

 formed prior to fertilization and even before the formation of 

 the polar bodies. 



The spermatozoon enters as a rule from a place diametrically 

 opposite the second polar mitotic figure, and at the metaphase 

 of the latter (PI. IL, Fig. 18.). The spermatozoon in the yolk 

 part of the ovum can always be recognized by the accompanying 

 sperm-asters ; were it not for this feature, the head of the sper- 

 matozoon might readily be mistaken for a yolk granule, since it 

 is only a little larger (2 ,«) than the latter. The spermatozoon 

 while still near the periphery of the egg resolves itself into eight 

 réfringent chromosomes floating in clear dense fluid (PI. IL, Fig. 

 19.). At this stage both the germ-nuclei, still wanting the 

 nuclear membrane, cannot at first sight be distinguished the one 

 from the other, but upon a more careful study it is found that 

 the egg-nucleus lies on the whole nearer the periphery than the 

 sperm-nucleus (PL IL, Fig. 19.). The sperm-nucleus comes next 

 to be provided with a thick nuclear membrane as in the egg- 

 nucleus. It then travels towards the centre of of the egg, and 

 there meets (PI. IL, Fig. 21.), and becomes apposed for a time to, 

 the egg-nucleus. The nuclear membrane dissolves away at the place 

 where the two nuclei come into contact with each other, so that at 

 one stage the cleavage nucleus thus formed assumes the form of an 

 hour-glass (PI. IL, Fig. 22.). In living ova the two germ-nuclei 

 can be seen as clear spots by the aid of low powers (50-60 

 diameters) ; by higher magnification, on the contrary, they cannot 

 be made out owing to the necessarily small depth of the objectives. 



