146 



CHAS. B. WILSON. 



by their almost complete separation ; tlie isthmus joining them 

 is very slender, and can be bent by a slight force. 



We thus have in these eggs a combination of strong spin- 

 ning activities with just the physical conditions best adapted 

 for rendering them manifest. These activities are more 

 profuse at this early stage than in any later period of de- 

 velopment. Whether the blastomeres are sensibly reduced 

 by this spinniog is difficult to determine, because they are 

 continually changing their shape at just the time when the 

 spinning is most profuse. In immature and abnormal eggs, 

 however, they often produce a perceptible diminution of the 

 blastomeres, and in such cases segmentation is usually un- 

 equal instead of equal, the third furrow producing four 

 micromeres and four macromeres. Subsequent division pro- 

 ceeds on the same plan, but the eggs are so weakened that 

 they seldom reach gastrulation. Tn this egg, as in the 

 Echinoderm, there is thus a marked difference between 

 normal and abnormal filose phenomena, as emphasised by 

 G. F. Andrews (5). Such abnormalities are of little use in 

 explaining normal cleavage, but may contain valuable sug- 

 gestions. Finally, we may note that these abnormal activities 

 are frequently carried so far that the egg spins out the entire 

 substance of the blastomeres, and goes to pieces during the 

 first segmentation. 



This first cleavage, therefore, occupies about fifteen minutes, 

 and is followed by an interval of rest lasting ten or twelve 

 minutes. 



This interval is occupied in the various phases of karyo- 

 kinesis ; the nuclei of the two blastomeres divide, the halves 

 separate and take a position at either end of the vertical 

 diameter of each blastomere, where they can be seen under a 

 low power. 



The second cleavage groove then appears at right angles 

 to the first, and divides the egg horizontally into four 

 blastomeres. These are not spherical, although they appear 

 so when viewed from the side (fig. 52). If the egg be rolled 

 between cover-glass and slide, the blastomeres are seen to be 



