OF ARTS AND SCIENCES. 47 



the first cleavage-nucleus, the first cleavage begins, not by a groove 

 on the external surface of the blastodisc, but by one on the internal 

 face. The early cleavages are all introduced by grooves running from 

 the inner towards the outer surface of the blastodisc. These intro- 

 ductory grooves, which have hitherto been entirely overlooked, but 

 which are very conspicuous in all the teleostean ova we have studied, 

 reach their greatest height (which is seldom more than one third the 

 thickness of the blastodisc) at the moment when the grooves from the 

 external surface begin. "We may call these inferior, in distinction 

 from the external, or superior grooves. The peculiarity of these 

 grooves is that they precede the appearance of the superior grooves, 

 and that they recede and rapidly disappear after the superior grooves 

 begin. The first of these inferior grooves begins as a shallow rounded 

 furrow, but usually culminates in a sharp angle. At the moment of 

 culmination we have several times been able to recognize a differen- 

 tiated vertical plane extending completely through the blastodisc, from 

 the edge of the inferior to the edge of the beginning superior groove. 

 The plane of division is then ali'eady established before an actual 

 separation begins. This foreshadowed plane of division is visible in 

 mounted preparations quite as early as any traces of division in the 

 chromatic elements of the nucleus. 



In five or six minutes after the inferior groove appears, the corre- 

 sponding superior groove begins ; and as it deepens, the inferior 

 groove closes up and becomes completely obliterated in the course of 

 eight or ten minutes. By this time the superior groove has cut fully 

 half-way through the blastodisc, and its walls begin to close together, 

 so that the deeper portion of the groove, seen in profile, has the form 

 of a vertical line, while its upper portion opens widely in a funnel- 

 like form. It is at this time that the second generation of nuclei be- 

 come visible. Gradually the linear portion of the groove lengthens, 

 upward by the closure of the funnel-shape space, and downward by 

 descending towards the inner surface. But the plane of division lias 

 not yet reached the floor of the blastodisc ; nor does it do this until 

 some minutes after two new amphiasters have formed, and the second 

 inferior groove is already well advanced (twenty-one minutes after 

 the first inferior groove). The floor is reached first of all at the 

 point Avherc the second inferior groove crosses the path of the first 

 superior groove. Nearer the margin of the blastodisc, where this is 

 continuous with the periblast, the floor is not reached until considerably 

 later. During all this we have looked in vain for any distinct trace 

 or shadowy indication of a lower stratum, which, according to Hoff- 



