CELL-DIVISION AND DEVELOPMENT. 



135 



constitution, chemical or physical, and yet, as E. B. Wilson 1 

 has shown, the cleavage of different ova of Amphioxus may 

 vary considerably, being in some cases radial, in others " spiral," 

 and in others even bilateral. The factors laid down by Sachs 

 represent only some of those which govern cell-cleavage. 



In this connection mention may be made of a set of phe- 

 nomena, first, I believe, pointed out by Rauber, 2 who groups 

 them together as instances of what he terms segment attrac- 

 tion, without, however, essaying to explain how this attraction 

 is effected. An excellent example of this phenomenon is 

 afforded by the segmenting egg of the Squid, Loligo, accord- 

 ing to the account given by Watase". 3 In this egg, which 

 undergoes what may be termed a meroblastic segmentation, 

 the first cleavage plane corresponds with the median longi- 

 tudinal plane of the adult animal, the egg being thus divided 

 at the first cleavage into a right and left half. The second 

 cleavage is at right angles to the first, while the third, repre- 

 sented by two planes, is practically parallel with the first. As 

 a result of these cleavages the proto- 

 plasmic pole of the egg presents the 

 appearance shown in Fig. 5, and 

 possesses, as may readily be seen, 

 a well-marked bilateral symmetry. 

 In the succeeding divisions the seg- 

 ment attraction becomes marked, 

 peculiarities affecting a cell or a 

 group of cells at one side of the 

 blastoderm being repeated in the 

 corresponding cell or group of cells 

 of the opposite side, even though a number of cells not show- 

 ing the peculiarities intervene. A concrete example of this 

 phenomenon, which Watase speaks of as analogous variation on 

 the two sides of the blastoderm, may be given. In an ovum 



1 E. B. Wilson : Amphioxus, and the Mosaic Theory of Development. Journ. 

 Morph., VIII, 1893. 



2 A. Rauber : Neue Grundlage zur Kenntniss der Zelle. Morph. Jahrb., VIII, 

 1883. 



3 S. Watase : Studies on Cephalopods. I. Cleavage of the Ovum. Journ. 

 Morph., IV, 1891. 



FIG. 5. (After Watase.) 



