DETELOPMENT OE TRACHINUS VIPEEA. 277 



takes place. The nuclei in tlie first two cells could not be dis- 

 tinctly made out ; but at about 11.15 a.m. a second furrow (fig. 3) 

 began to appear at rigbt angles to the first. Eacb furrow begins in 

 the centre and extends outwards, deepening more rapidly at its 

 origin, so that the farrow is complete at the inner margin of the 

 cell before it has reached the outer (fig. 5). When the furrow 

 reaches (about 11.45 a.m.) the outer margin, the latter becomes 

 indented, and the four-celled stage is then fully marked out as 

 shown ia fig. 4 ; and two minutes later nuclei appear, but are 

 only faintly visible in this stage. At 12.50 p.m. the nuclei had 

 completely disappeared, and the blastodisk had become almost 

 square. At 1.10 the segmentation-furrows were beginning to 

 form in the same general direction as the first furrow, but some- 

 what at an angle with it (fig. 5). At 1.35 the eight-celled stage 

 was completed, as in fig. 6, with the nuclei again visible. By 

 2.30 the nuclei had again completely disappeared ; and at 2.40 

 new farrows made their appearance at right angles to the pre- 

 vious (fig. 6). At 2.55 the sixteen-cell stage was complete, but 

 the nuclei did not appear until ten minutes later, and in this 

 stage are very distinct (fig. 7). The nuclei had again disappeared 

 at 3.35 ; and from this time the segmentation is not quite regular. 

 The first cells to divide are the two centre ones on each sic'.e, 

 nest the corner cells, and afterwards the four central cells ; but 

 the whole process only occupies 15 to 20 minutes, and this time 

 the four central cells no longer divide vertically, but in a hori- 

 zontal plane so as to form two layers. From this point ifc is 

 diflBcult to follow the process of segmentation, the outlines of the 

 cells being too confused through overlapping. The disk is now 

 almost circular, and has a diameter of about "73 millim. Two 

 hours later segmentation had progressed rapidly, particularly in 

 the centre, where the cells were at least four rows deep. About 

 this time free nuclei make their appearance in the " nuclear 

 zone," and free cell-formation takes place. In two hours' time a 

 first row of these free cells has been formed round the disk, and 

 there are a few isolated cells of a second row (fig. 8) . A little 

 later a partial side view of the egg showed that the cells of the 

 "nuclear zone" had pushed their way partly under the outer 

 edge of the blastodisk. Three hours later it is found that seg- 

 mentation has proceeded ; but there is still no increase in the 

 diameter of the blastodisk, which is now quite circular. The 

 disk has now the appearance presented in fig. 9. There are three 



