42 



CHORDATE ORGANIZATION 



following stormy weather. Fertilization is external and development 

 then occurs free in the water. Numerous eggs are produced and they are 

 small but yolky. Complex flowing movements take place in them after 

 fertilization, and cleavage is then rapid and complete, producing a 

 blastula composed of a dome of somewhat smaller and a floor of rather 

 larger cells (Fig. 17). These latter then invaginate to make the archen- 

 teron, opening by a wide blastopore, which later becomes the anus. 

 At about this stage the gastrula becomes covered with flagella, by 

 which it rotates within the egg case. 



Fig. 17. Three stages in the development of amphioxus as seen in stained 



preparations. 

 a, the blastula; b, early, and C, later gastrula. 



The creature now elongates and its dorsal side flattens and eventu- 

 ally sinks in to form the neural tube (Fig. 18 a). At about this time 

 the dorsal side of the inner layer begins to fold near to the front end, 

 in such a way as to make a pair of lateral pouches. The walls of these 

 pouches are the future mesoderm and the cavity is the coelom. As in 

 other early chordates, therefore, the coelom is continuous at first 

 with the archenteron. The roof of the archenteron also arches up 

 dorsally and forms the notochord, the gut wall being completed by the 

 approximation of the edges of the remaining portion of the inner 

 layer, which is now the definitive gut wall or endoderm. 



The analysis of the processes of development now enables us to say 

 something of the forces by which these formative foldings and cell 

 movements are produced. The formation of the neural tube, meso- 

 derm and notochord and the completion of the gut roof all involve an 

 upward movement of cells towards the mid-dorsal line. This process 

 of 'convergence' is a very marked feature of the development of all 

 chordates (Young, 1957, p. 609). 



As the animal elongates, further mesodermal pouches are produced, 

 each separating completely from the endoderm and from its neigh- 



