EARLY DEVELOPMENT OF AMPHIOXUS. 45 



end. The somites appear as bilaterally symmetrical structures, but when 

 five or six pairs have arisen the symmetry is disturbed since each somite 

 on the right comes to lie a little behind its fellow on the left thus giving 

 an alternation which is carried on into the adult. 



Only the first few somites develop with enteroccelic cavities, the remainder 

 originating as solid structures although the cells are arranged radially around 

 a central point. However, the solid ones subsequently acquire cavities. 

 The enteroccel has been regarded as an indication of a primitive character, 

 since in the higher animals the somites do not contain any cavities derived 

 from the gut cavity but arise as solid structures. On the other hand the 

 solid somites may indicate the primitive condition and the appearance of 

 enteroccelic cavities may be a secondary character in Amphioxus. 



The rudiment of the notocord, mentioned previously, which is composed 

 of the entodermal cells immediately ventral to the neural tube and between 

 the two mesodermal outgrowths, extending from the cephalic extremity of 

 the embryo to the blastoporal region, requires brief attention. While the 

 mesodermal rudiments are being cut off from the parent entoderm the 

 notocordal cells become rearranged into a compact rod-like structure lying 

 between the somites of the two sides (Fig. 22, d). As the somites enlarge 

 this rod is constricted from the adjacent entoderm, which then closes across 

 the top of the gut cavity, and occupies its definitive position ventral to the 

 neural tube. Clearly the notocord in Amphioxusj)riginates from entoderm. 

 As the embryo continues to grow in length the notocord too is lengthened by 

 the addition of cells to its caudal end in the region of the neurenteric canal. 



Continued development of the mesodermal somites comprises their 

 farther intrusion between ectoderm and entoderm and changes in their 

 component cells. When first formed, the somites are composed of columnar 

 or cuboidal epithelial cells in a single layer surrounding the central cavity if 

 present, or, if the enteroccel is absent, radiating from a common center (Figs. 

 23 and 24). The somites are block-like in shape and located lateral to the 

 developing notocord and neural tube. The changes to be described begin 

 in the anterior somites and, in accordance with the principle of growth 

 already mentioned, progress from there backward. The cavity in the 

 somite becomes larger and the surrounding cells become flatter. With the en- 

 largement of the cavity the ventral portion of the somite extends ventrally 

 between ectoderm and entoderm (Fig. 22, d). It seems that the whole 

 structure becomes dilated in the direction of least resistance. The outer por- 

 tion of the wall is apposed to ectoderm and is called the somatic or parietal 

 mesoderm; the inner layer is in contact with entoderm and is spoken of as 

 splanchnic or visceral mesoderm. The dilated cavity is the codomic space 

 ^(Figs. 22 and 25). Continued ventral extension brings the dilating struc- 



