No. 3.] AMPHIOXUS AND THE MOSAIC THEORY. 593 



cleavage ; its ultimate direction is determined by the position 

 accidentally assumed by the blastomere after its displacement. 

 Thus in Fig. 106 the two blastomeres must have been slightly 

 rotated on one another in a vertical plane nearly parallel to 

 that of the first cleavage. In Fig. 107 the left blastomere was 

 rotated in a vertical plane, the right in a horizontal plane at 

 right angles to the first. 



Let us now consider the form of cleavage. In this regard 

 the double embryos show a series of for^ns intermediate between 

 a Jialf-cleavage (like that of an isolated ^ blastomere) and a 

 cleavage-half {like one-half the norvtal cleavage). Several such 

 forms are shown in Plate XXXVI. In Fig. 108 the left half 

 consists of two macromeres with two micromeres lying above 

 them ; that is, it is a typical cleavage-half. The right half, on 

 the contrary, consists of four equal cells, lying in a horizontal 

 plane ; the third cleavage has here become vertical instead of 

 horizontal, and equal instead of unequal. In Fig. 1 10 the third 

 cleavage is vertical and equal on both sides (typical double 

 4-celled stage). Fig. 109 is intermediate as regards the planes 

 of cleavage, but all the cells are equal. 



Analogous and still more interesting transitional forms occur 

 in the following stage. The most striking of these forms is 

 shown in Fig. 114 (which represents the individual that first 

 called my attention to the importance of the double cleavage). 

 The nuclear spindles (all distinctly visible in the preparation) 

 leave no doubt as to the origin of the cells, and demonstrate 

 the mode of transition. In each half four micromeres are in 

 process of formation. Two of them (those in the upper half 

 of each twin) agree with the normal cleavage, arising by the 

 vertical fission of a parent micromere of the (common) 8-celled 

 stage. The other two are in process of separate formation 

 from the two corresponding macromeres. Thus, the upper 

 half of the figure follows the normal mode of cleavage, while 

 the lower half shows the regular double cleavage. Inspection of 

 the figure will show that the disturbance in the form of 

 cleavage showed itself at the third (common) division, which, 

 instead of being as usual equatorial in all of the four blasto- 

 meres, became vertical in two of them. In the true double 



