398 DEVELOPMENT OF FROG AND OTHER VERTEBRATA 



Development of Amphioxus (Branchiostoma lanceolatum) 



Reference to the table of classification on page 36 will show the 

 position of amphioxus as a member of the Phylum Chordafa. The 

 adult animal (Fig. 4 B, p. 13) does not exceed two and one-half 

 inches in length. It hves in shallow water, burrowing in the sand 

 or swimming for short distances by fish-like movements of the fins 

 and body. The mouth is located at the anterior end, surrounded by 

 a fringe of oral cirri, while the atrial pore, or opening by which the 

 excurrent respiratory water leaves the body, is located at about 

 the position of the anus in most fishes. The true a7)us opens 

 internally into the atrial cavity. The tubular central nervous 

 system, with its dorsal position, the 7iotochord, gill slits, and other 

 structures mark the animal as unquestionably a chordate. It is 

 particularly interesting because its structure is suggestive of an 

 organization antecedent to that of the vertebrates. 



The sexes are separate in amphioxus, and the ova and sperma- 

 tozoa are discharged through the atriopore into the water where 

 fertilization occurs. Cleavage (Fig. 209 and cf. Fig. 213) pro- 

 duces a symmetrical blastula, with the animal hemisphere com- 

 posed of small cells and with somewhat larger cells toward the 

 vegetative pole. Gastrulation occurs by a simple invagination, 

 as in other cases where the egg has only a small amount of yolk 

 distributed throughout the cytoplasm (homolecithal). The two- 

 layered sac that is thus formed elongates as the blastopore becomes 

 reduced in size, while a differentiation of the ectoderm cells marks 

 the future nervous system on the dorsal surface of the body. 

 Transverse sections of this stage show that the ectoderm and endo- 

 derm are being further differentiated into the beginnings of the 

 nervous system, the notochord and the ccelome, while the archenteron 

 is becoming converted into the digestive tract. In longitudinal and 

 transverse sections at a later stage, the typical chordate organiza- 

 tion is apparent. Amphioxus, therefore, develops Ijy cell division 

 and differentiation from a single cell to a many-celled organism, 

 like all other metazoa. The sequence of events in this development 

 are well known, but the causes of the many steps, as, for example, 

 the conditions that bring about gastrulation, notochord forma- 

 tion or the differentiation of the neural plate at a particular stage 

 of the development, remain for the most part unsolved problems 

 for the experimental embryologist. 



