386 WILSON. [Vol. VI, 



three parts, the middle of which assumes an obHque position 

 (/), and constitutes the well-known " cross-furrow " (" Brech- 

 ungslinie " of Rauber), which occurs in so many eggs at this 

 stage. Meanwhile, through a slight displacement of the four 

 blastomeres, a cross-furrow appears on the upper pole, much 

 shorter than the lower one and at right angles to it. At the 

 completion of the division, therefore, the four blastomeres have 

 the arrangement shown in Fig. 5. 



This arrangement is of great interest with reference to .the 

 law of cleavage, and is of the utmost practical importance in 

 the orientation. At the upper pole B and D are separated, 

 while A and C are in contact along the short cross-furrow, 

 which passes from left to right. On the lower hemisphere, this 

 arrangement is reversed, B and D being in contact along the 

 cross-furrow, zvhich passes front right to left (viewed from the 

 upper pole), while A and C are separated. The significance of 

 this arrangement will be discussed in Part X, and I will here 

 only point out its importance as a means of orientation. Owing 

 to the fact that the four primary entomeres do not divide, the 

 lower cross-furrow remains unaltered up to a stage when the 

 prototroch is in full activity, the mouth, eyes, and mesoblast- 

 bands have been formed, and the segmentation of the body has 

 become apparent (see PI. XX, Figs. 85, Z6, and preceding fig- 

 ures). Thus the relations of the four-celled stage to the tro- 

 chophore are, I believe, unmistakable, and all the intermediate 

 stages may be oriented with perfect certainty by means of the 

 position of the furrow, taken in connection with the size and 

 arrangement of the cells. Furthermore, the slight acceleration 

 in the division of the smaller of the first two blastomeres is a 

 safeguard against error in determining the relations of the four- 

 telled stage to the first two cleavage-planes. I believe, there- 

 fore, that I can state with absolute certainty that the second 

 furrow coincides with the median plane of the larval and adult 

 bodies. 



Side views of the four-celled stage (Fig. 6) show that the dif- 

 ference in size between the large blastomere and the others is 

 much greater than appears from the polar view, since its verti- 

 cal diameter is greater than that of the others. The oil-drops 

 lie far down in the blastomeres, the upper portions of which still 

 have the same structure as in the unsegmented ^gg. 



