236 C. O. WHITMAN. 



the cleavage, tlie rhig-rays of each pule become more and more 

 freble. The calotte a])proaches that side of the ring which lies 

 nearest the plane of the coming cleavage (fig. 14), giving the 

 ring the form of a semilunar spot. The calotte generally remains 

 circular; but when very small, sometimes stretches and forms a 

 mere line at right angles to the lirst cleavage-plaue. The 

 approach of the calotte to one side of the ring does not interrupt 

 the continuity of the latter as a profile view (fig. 13) proves. At 

 this time the aboral ring-disc is reduced to a mere point, with 

 scarcely perceptible rays. Pigs. 10 — 13 show that although the 

 ring and rays begin later on the aboral pole, they pass through 

 their different ])hapes more rapidly than those of the oral pole. 

 The rays are well seen on eggs hardened in chromic acid. 



I will now pass to the consideration of the changes taking 

 place within the egg during the ring-peiiod. 



Pig. 66 (PI. Xlli) represents theinci]nent formation of the first 

 ring (p.r.), and corresponds nearly with fig. 9, PI. XII (1 h. 30min.) 

 The female pronucleus has advanced a little towards the centre 

 of the egg. The space between it and the polar globules is still 

 free from large yolk-spheres. In this case the action of the 

 acid was weak, and that of the carmine correspondingly strong, 

 the entire pronucleus being deeply coloured. 



Fronucleoli. — On the inner side of the pronucleus are seen 

 two small, highly refractive corpuscles, in close apposition, which 

 together measure 01 mm. dm. These small bodies, which I shall 

 CdiW female pronncleoli {i.pnl) are sharjily defined, homogeneous, 

 and more deeply coloured than the nucleo])lasm (fig. 66, 1)). 



The male pronucleus (m. pn) is now near the centre of the 

 t^g, and shows in its centre a single oval-eliiptical body, of the 

 same nature as the female pronucleoli. 



This body is the male piwmcieo/ns {?/i. pnL). Both ])ronuclei 

 are surrounded with radial lines, and their longest axes lie in the 

 main axis of the egg. 



In some eggs of the same date, I found two ])ronucleolar bodies 

 in the central nucleus (fig. 66, P), lying at some distance from 

 each other. The one lying nearest the oral i)ole is composed of 

 two parts {/. pnl), and corresponds in size to the two female 

 })ronucltoli (D). The axes of such nuclei lie sometimes j)aralhl 

 with, somefimes inclined to the main axis of, the e^g (/'), and 

 the radial lines are very faintly, or not at all, expressed. In 

 these eggs only one nucleus was found. They rej)resent probably 

 a stage intermediate between fig. 66, C, and tig. 67 G. 



Pifteen minutes aftrr the appearance of the first ring (1 h. 

 45 min.), the egg reaches the condition represented in iig. 67. 

 The up|)rr ring ( p r) is here very distinct, but I am unable to 

 distinguish the lower ring, although it is generally present at 



