:34 ART. 1. — T. FUJITA ! ON THE FORMATION 



^Nevertheless, it should be borne in mind, that cleavage commences 

 invariably from one or the other of the posterior blastomeres or 

 its descendants. 



2. After the second cleavage the opposite quadrants of the 

 blastomeres usually come in jaxtaposition forming the cross furrow 

 between. Kofoid has made the statement that the quadrants 

 forming the furrow at the two opposite poles differ with species 

 according to the amount of the deutoplasm in the egg. This 

 does not hold good at least in the present cases. Siphonaria and 

 Aplysia perhaps represent two extremes with regard to the 

 quantity of the deutoplasm. Nevertheless, as has been seen, the 

 cross furrow in these two species is formed at both poles by the 

 same quadrants. 



3. During the cleavage of the egg, the daughter-cells are 

 disposed in a spiral form, that is, the individual cells shift 

 their respective positions either toward the right or the left of 

 their original positions. Such a spiral arrangement or symmetry 

 is retained for some time. It is then abruptly transformed into 

 a bilateral symmetry. This important phenomenon, so tar as I 

 am aware, has been interpreted as due to the rearrangement of 

 the component cells. It is true that in an advanced stage the 

 disposition of the cells becomes altered ; but there is another im- 

 portant factor which necessitates such a transformation of the 

 body symmetry. The factor is the existence of certain new cells 

 propagated from the fourth generation of ectomere-cells. Of the 

 «iuartet which forms this generation, the two posterior cells give 

 rise in a peculiar way to the daughter cells which I have called 

 the bilateral cells. These play an important role in bringing 

 about the change of the -body symmetry, which seems to take 

 place in a comparatively short space of time. The period of 



