MUSEUM OF COMPAEATIVE ZOOLOGY. 471 



sists when, soon after, the yolk nucleus becomes faintly outlined and 

 disappears. 



As I have elsewhere indicated, it is probable that Fol ('75^ PL VII. 

 Fig. 2, and PI. VIII. Fig. 2) saw and figured for Pteropoda, without com- 

 prehending its true significance, the male pronucleus, both some time be- 

 fore and also when it was about to join the female pronucleus, in the 

 former case as the centre of a well-expressed aster. 



0. Hertwig (75, pp. 378 - 398, Taf. XI.) was the first to definitely 

 connect one of the pronuclei (Spermakern) with a spermatozoon. In 

 from five to ten minutes after artificial fertilization of the eggs of the 

 sea-urchin there appears near the surface a small clear space from which 

 the yolk granules have disappeared. This space increases a little in size, 

 and at the same time the neighboring yolk granules assume a radial 

 arrangement about it as a centre ; at first limited to its immediate 

 vicinity, but gradually becoming more extensive and more distinct. A 

 small homogeneous body makes its appearance in this space, from which 

 it only slightly diflPers in its refractive power. Sometimes a delicate line 

 was seen stretching from this body to the periphery of the yolk, whence 

 it continued into the perivitelline space as a fine thread. This radial 

 figure migrates rapidly (requiring only about five minutes) from the 

 periphery to near the centre of the Q^g ; here the corpuscle encounters 

 the " egg nucleus " (female pronucleus), which has meantime slowly 

 approached the stellate figure. The egg nucleus has a diameter of 13/x, ; 

 the corpuscle, of ifx. The nucleus now undergoes a slight amoeboid 

 change of form, both structures become less distinct, and the smaller 

 finally disappears. A little later the limitation of the egg nucleus again 

 becomes distinct, but the smaller body is not to be seen. The nucleus 

 is larger than before, and of spherical form. Meanwhile the stellate 

 figure, in which the Qgg nucleus has now come to lie, has increased in 

 extent till its rays reach nearly to the periphery of the yolk on all 

 sides. 



The use of osmic acid and Beale's carmine confirms the results of these 

 observations on living eggs. The stellate figure is, however, less conspic- 

 uous than in the fresh condition. By this treatment it is found that 

 both the egg nucleus and the central corpuscle of the stellate figure be- 

 come deeply stained. This warrants the conclusion that both consist of 

 nuclear substance. The corpuscle is a little more intensely colored than 

 the egg nucleus, a condition to be accounted for by the more compact 

 condition of its substance. Furthermore, stages in which the two nuclear 

 structures are in contact, and later such as show only a single nuclear 



