THE DEVELOPMENT OF AMPHIOXUS 11 



greater freedom from yolk of this region of the embryo, and 

 this has an important bearing upon subsequent development. 

 The egg axis is at an angle of roughly 30 to the antero-posterior 

 axis of the embryo : that is, the animal pole of the egg becomes 

 ant ero- ventral, the vegetal pole postero-dorsal. It is quite 

 probable that this bilateral symmetry, and certain that the 

 polarity of the egg, is determined while it is still within the 

 ovary; it gains visible expression through the assumption of a 

 peripheral position by the primary oocyte nucleus and the 

 eccentricity of the accompanying clear protoplasmic region. 

 The egg of Amphioxus seems to resemble that of the Echinoderm 

 rather than those of most other forms, in that the direction of 

 this eccentricity of the nucleus, i.e., animal pole, is toward the 

 region of attachment in the ovarian epithelium. 



2. Cleavage 



The cleavage of Amphioxus is total and unequal and, 

 though subject to much variation, it is in many eggs quite 

 regular. It is generally believed that the Amphioxus type of 

 cleavage is primitive among Chordates; and that the cleavage 

 modes of Craniates are to be derived from it, the chief cause of 

 modification being the accumulation of yolk. The first division 

 of the egg occurs about an hour after entrance of the sperm, 

 the second about an hour after the first, and subsequent 

 divisions about every fifteen or twenty minutes. The plane 

 of the first cleavage as indicated by the position of the first 

 cleavage spindle, is median (cleavage furrow meridional), 

 dividing the egg into exactly similar right and left halves, 

 which become the right and left halves of the embryo. The 

 second cleavage furrow is also meridional and the plane at 

 right angles to the first. Cleavage becomes unequal with this 

 division for the two cells are divided into two smaller antero- 

 dorsal, and two larger post ero- ventral cells, symmetrically 

 arranged on each side of the median plane (Fig. 5, A). 



The third cleavage is at right angles to the first two and the 

 furrow equatorial, or rather latitudinal for again the division 



