364 



THE NEW INDIVIDUAL 



Part IV 



so little yolk that their processes can be clearly observed through the micro- 

 scope. The pattern of development is, on one hand, similar to that of hydra 

 and the starfish, animals far older in evolutionary history, and, on the other, 

 similar to that of the vertebrates that are much younger in evolution. 



Early Development. The early development of amphioxus proceeds on a 



(5) Ground 

 protoplasm 



(2) Grey 

 crescent 



Fig. 19.3. Fertilized egg of the tunicate 

 Styela. A crescent of yellowish protoplasm 

 (yellow crescent) becomes the posterior 

 end and a crescent of grayish protoplasm 

 (f^ray crescent) becomes the anterior end 

 of the embryo. Even at fertilization the 

 cytoplasm of the egg becomes arranged 

 for particular destinations in the develop- 

 ing animal. (After Conklin. Courtesy, 

 Shumway: Vertebrate Zoology, ed. 4. 

 New York, J. Wiley & Sons, 1942.) 



(3) Grey yolk 



plan followed in essentials by all vertebrates. Development is ordinarily a con- 

 tinuous process. It includes stages such as cleavage, blastulation, and gastrula- 

 tion that blend into one another. 



Cleavage and Blastulation. The first cleavage begins as a slight depres- 

 sion at the animal pole. This deepens and lengthens into a constriction which 

 divides the egg into the first two cells representing the right and left halves of 

 the new animal (Fig. 19.5). The second cleavage also begins at the animal 

 pole, at right angles to the first. The third one is at right angles to the first two 



Coelom 



Brown funnel 



Nofochord 



Brain 



Spinal cord 



Myomere 



Dorsal fin 



Cirrus 

 Oral hood 



Caudal fin 

 I 



Mouth 



Branchial 

 clefts 



Gonad 



Atnum 



?> 



Anus 



Fig. 19.4. An adult amphioxus with part of the body wall removed from the left 

 side. The essentials to be noted are the relative positions of the spinal cord, note- 

 chord and alimentary canal. Amphioxus is generally regarded as an ancient ances- 

 tor of the vertebrates. The fundamental plan of its development is followed in all 

 of them. Adults are two inches long. (Courtesy, Rand: The Chordates. Philadel- 

 phia, The Blakiston Co., 1950.) 



