6 Art. 3. — Xaohide Yatsu : 



meet.' At any rate prior to the first cleavage, there takes place a. 

 change in the distribution of the ectoplasm: it thickens consider- 

 ably near the macromere pole,' while at tl)e opposite pole it thins 

 out a great deal (PI. I, Figs. 9 and 10). Cleavage goes on, as has 

 been observed by a good many investigators. Sometimes the 

 cleavage fuiTOw is bent sHglitly to one side near its completion 

 (PL I, Fig. 11). It should liere l)e especially mentioned tliat the 

 cleavage is not strictly unihiteral, contrary to Ziegler's observa- 

 tion ('98. p. 41; '03 p. 159 and his diagram Fig. 7), and also to 

 Rhumbler's opinion based upon Zieglers results. A sliallow 

 depression is always present at the micromere pole, as is seen in. 

 the sketches drawn one upon another at different periods (PI. I, 

 Fig. 11). And it will be also noted that the top of each blas- 

 tomere becomes more rounded. The lateral elongation is almost 

 nil. The rate of tlie cleavage from the macromere pole is <S-19/i 

 per minute, 17/< on an average, at a room temperature of G5°-07°F. 

 The rate of cleavage from tlie opposite pole is a little slowei". 13/>- 

 per minute on an average. 



To supplement Ziegleu's observations, the j^rocess uf cleavage 

 and especially the "cleavage head" (Furchenkopf) Avill be des- 

 cribed in some detail. In the beginning a slight depression ap- 

 pears near the polocytes. Its optical section is shown in Fig. 12. 

 (PL II). Here is a pair of prominences in the outer homogeneous 

 layer (cf. Andrews '98). Sometimes they are continuous, forming, 

 a bridge over the now deepening furrow.^ Sometimes there is one 

 process on one side and two on the other (PL IL Fig. 12) (rf. 

 Kleinenbeeg '72 p. 49., Taf, 4, Fig 4; Wager '09 p. 23, PL III, 

 Fig. 23a). Around the process a fine display of spinning activity 

 is visible. At the bottom of the cleavage depression are rays in tlie 

 homogeneous laver. 



1 As is seen in Figs. 10 and 11. the polocytes ai-e usually situât h1 at some distance from- 

 the cleavage furrow. Whether the egg-nucleus or cleavage-nucleus moves a little from the spot 

 ■where the polocytes liave been formed, or whether the polocytes are transported by some means, 

 is not certain. But the latter alternative seems to be the more probable one. 



2 Macromere pole = vegetative pole of Hatschek (Korschklt and Hkider 'OJ p. 24). 

 Throughout the i)lates the macromere pole is above, and the micromere pole below. 



3 The bridge is not so distinct as was obsei-ved by Tannreüther in the Hi/dra egg, where- 

 yolk granules were seen to pass from one blastomere to the other ('08 p. 2ö7). 



