No. 3.] AMPHIOXUS AND THE MOSAIC THEORY. 603 



type. The fact which I have elsewhere described (35), that in 

 some animals the bilaterality does not appear at the beginning 

 (in annelids not until the 38-celled stage) indicates that this 

 process has taken place by a gradual shifting backwards in the 

 ontogeny of the adult bilaterality, which thus became projected, 

 as it were, upon the cleavage-stages. 



Accepting these conclusions, the variable cleavage of Aviphi- 

 oxits is a very interesting case ; for we here observe, as it were, 

 a conflict between an hereditary tendency not yet firmly 

 established, and mechanical conditions that are, in a measure, 

 opposed to it. It is impossible not to recognize a distinct ten- 

 dency towards bilaterality in almost all forms of the cleavage, 

 but it is often disturbed by displacements of the blastomeres. 

 In some cases the displacements are very slight, and the bilat- 

 eral form is scarcely modified, but every transition exists 

 (Figs. 22, 23, 24) up to a condition in which the bilaterality is 

 completel)' lost, and the cells are arranged precisely as in the 

 true spiral cleavage (Figs. 31, 32). In the latter case the cells 

 are arranged with greatest economy of space, in accordance 

 with Berthold's principle, and a careful study of the various 

 transitional forms shows that the displacements of the indi- 

 vidual blastomeres show, in a large proportion of cases, a dis- 

 tinctly recognizable conformity to the same principle, as may 

 be seen by a comparison of the dividing micromeres in Figs. 22, 

 23, 24, for example, with the corresponding cells in Fig. 19. 

 These displacements, it should be noted, do not, in such cases, 

 take place subsequent to the division of the blastomeres, but 

 during the division itself, and are, probably, in many cases due 

 to modifications of form caused by pressure. There can be no 

 doubt that many details of egg-cleavage, in general, are directly 

 determined by this influence, the cell elongating in the direc- 

 tion of least pressure, and the spindle placing itself, with its 

 long axis, in the same direction. This may be clearly made 

 out, for example, in many of the detailed figures given of the 

 cleavage-stages of Nereis (No. 35), for example, in Figs. 13, 

 16, 19, 22, etc. It is to this cause, as I believe, that we must 

 ascribe many, if not all, of the variations in Amphioxics. 

 Their first origin is probably to be sought in slight, accidental 



