222 BULLETIN OF THE 



latter exhibits no indication of nucleoli.* The egg was treated with 

 acetic acid, and subsequently stained in Beale's carmine. 



III. Cleavage. 



The living egg has been followed in its changes through the formation 

 of two polar globules and the subsequent growth and approximation of 

 two nuclear bodies, the so-called male and female pronuclei. 



These nuclei remain near the surface at the animal pole. They may 

 be distinguished in the living egg as two distinct bodies up to within a 

 short time previous to the rapid changes of the first cleavage. Shortly 

 before that event, their outlines are no longer discoverable in the fresh 

 egg. The region remains more clear, but all that can be distinguished 

 is a more or less circular, ill-defined area, which is less opaque than the 

 surrounding portions of the vitellus. After a few moments, this area 

 grows less distinct. It finally appears elongated. Very soon this 

 lengthening has resulted in two light spots, which are inconspicuous 

 at first, but which increase in size and distinctness, and at length 

 become oval (Fig. 31). The long axes of the ovals are so directed that, 

 if prolonged, they would meet a little way beyond the animal pole of 

 the yolk. 



During the earlier part of this series of proceedings, — viz. soon after 

 the meeting of the two pronuclei, — there is usually an accumulation of 

 transparent protoplasm about the animal pole ; or, in other words, the 

 granules of the vitellus vanish from this portion of the yolk, leav- 

 ing sometimes a very thick superficial layer (Fig. 70^), at other times 

 only a comparatively thin covering, of clear protoplasm. This surface 

 layer may occasionally be traced quite around the yolk ; in other cases, 

 it can be followed for only a short distance from the animal pole. 



If the outline of the egg be carefully watched about the time of 

 the formation of the two new light spots, it will be seen gradually 

 to lengthen in a direction corresponding to the line which joins the 

 spots (Fig. 31). As the latter enlarge, the lengthening increases, though 

 not very conspicuously. At length a slight flattening of the surface 

 appears just under the polar globules. This finally changes into a very 

 shallow depression (Fig. 37), which grows deeper (Fig. 32), and becomes 

 angular. If the yolk be viewed along the polar axis at this time, it will 



* It is possible that the body I have called male pronucleus (mpn ? ) may repre- 

 sent the female pronucleus, in which event the structure marked fpn ? might be only 

 the "area " of the deep star of the second archiamphiaster. 



I 



