234' C. O. AVHITMAN, 



the mninont of its liberation, and it is plain to see what part the 

 amphiaster takes in its formation (cp). 



Polar Activit7/.—V^\\\\Q the phenomena thus far described — 

 polar figure, pellucid spot, and polar globules — have been con- 

 fined to one pole, those which are to follow are repeated, with 

 some differences, on both })oles. A short period of unipolar 

 activity is succeeded by a long ])eriod of bipolar activity which 

 extends through the cleavage stages. In the latter period the 

 contrast between the two poles is still maintained : for the pole 

 thus far active, still asserts its pre-eminence by taking the lead in 

 actions that repeat themselves later and more sluggishly on the 

 opposite pole. 



It is as if one pole w^as trying to mimic the performances of 

 the other. The more active pole is further distinguished by being 

 specifically lighter than the opposite pole, so that, with the excep- 

 tion of the short time during which the first polar globule is being 

 eliminated, this pole is always u|)permost. As this pole corres- 

 ponds to the anterior end of the future embryo — the pellucid 

 spot marking very nearly the position of the future mouth — it 

 may be called the oral pole, and the opposite, ike ahoral pole. 

 Thus, the main axis of the egg corresponds to the longitudinal 

 axis of the embryo. 



d. Formation of Polar Rings and Pronuclei. — The ring- 

 phenomenon, Uke the constriction accompanying the exit of the 

 polar globules, is peculiar to the egg of Clepsine, nothing of the 

 kind having as yet been found on the egg of any other animal. 



The first to describe the ])olar rings was Grube (ttV'ttt)' 

 The manner of their formation, however, was entirely misunder- 

 itood. He suj)posed that the white ])olar figure {" Poltleck ") 

 enlarged and became the polar ring. This mistake was corrected 

 by Robin (ttt~Vot)j ^'l^f* J'^^ followed the ring-phenomenon from 

 beginning to end, and described its minutest details with great 

 accuracy. So far as an outward descrij)tion goes, I have but 

 little to add to the observations of Robin ; but I shall be under 

 the necessity of giving a brief account of the external appearance 

 in order to bring them into relation with internal changes, of which 

 Robin was, of course, not cognizant, as he made no use of sections. 



Immediately after the appearance of the second polar globule, 

 the pellucid spot, which marks the place of its exit, is still 

 visible. A section of the egg at this time (PI. XIII, fig. G5, 1 h. 

 45 min.) shows the polar globules (p.g.) lyi'ig iu a slight depres- 

 sion, caused by the action of the acid. Beneath the globules 

 there is a circular space, free from deutoplasm. This space, 

 open towards the globules, is filled with a very fine granular 

 substance, which has the lead-grey tinge, chararteristie of the 

 germinal vesicle whieli has been treated with osmic acid. The 



