384 WILSON. [Vol. VI. 



occasionally containing a few deutoplasm-spheres. The proc- 

 esses of the opposing margins first come into contact near the 

 middle of the furrow {i.e. near the upper pole), and are gradu- 

 ally flattened out against one another, so that the central portion 

 of the furrow becomes a straight, even line. From this initial 

 point of union, the closure of the furrow proceeds in like man- 

 ner in each direction, around to the opposite side of the vitellus, 

 after which the two blastomeres become pressed together, and 

 the Q%% enters upon a resting period of fifteen to twenty min- 

 utes. The entire cleavage occupies about five minutes. 



The behavior of the vitellus during the division gives the 

 impression that the protoplasm is a viscid, tenacious mass, like 

 pitch, which is pulled apart into two masses, against a consider- 

 able resistance. When, for example, the furrow has cut nearly 

 through the vitellus, a narrow bridge of protoplasm is often left 

 at the lower pole. As this bridge is cut through, the deuto- 

 plasm-spheres within its substance are drawn out into an oval 

 shape, as if the viscid mass were being dragged out by the ends. 

 The formation of the pseudopodial processes shows, however, 

 that the protoplasm is actively at work, and that it is not pas- 

 sively sundered by attractive forces, emanating from a central 

 point within each incipient blastomere. It is worthy of note, 

 furthermore, that the edges of the furrow appear to be pressed 

 together with considerable force, since the pseudopodial proc- 

 esses may be seen to flatten out against each other, as if urged 

 from the rear. While this operation is taking place on the upper 

 portion of the furrow, however, the lower portion is still cutting 

 down through the vitellus, — a fact which likewise has some 

 bearing on the theory of attractive centres. As the cleavage- 

 furrow passes downward, it seems to carry with it the anterior 

 third of the polar area which disappears between the two blasto- 

 meres as they flatten together. The remaining two-thirds still 

 persists in the larger blastomere as an irregular, clear space. 

 In this space I have been able to see, in some specimens, but 

 not in aU, an irregular, coarsely granular mass, which shows a 

 marked contrast to the surrounding protoplasm, although it has 

 no definite boundary. This mass lies somewhat to the left of 

 the middle line, and at the next cleavage passes into the left 

 posterior macromere {cf. Figs. 3 and 5). Its later history I have 

 not followed with sufficient care, but I am tolerably certain that 



