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,^ 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 Qgg of the Squid, Loligo, accord- 

 ing to the account given by Watase.^ 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 tgg 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 Watas6 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, /ourn. 

 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.) 



