THE DEVELOPMENT OF AMPHIOXUS 3 



ary modifications of development appear comparatively late in 

 its course, and many of them are obviously correlated with 

 rather unusual habits and activities. 



The embryological history of Amphioxus is described here, 

 then, not only because of the morphological importance of this 

 form, but because of its embryological primitiveness and the 

 diagrammatic simplicity of its early stages, which will aid in 

 understanding the complicated development of the other Chor- 

 dates described. Here we have Chordate development re- 

 duced to its simplest terms. A fair knowledge of the anatomy 

 of Amphioxus is presupposed. 



The whole developmental history of Amphioxus falls quite 

 naturally into four general periods. 



I. PKODUCTION OF THE GERM CELLS. This includes the 

 formation of the germ cells, oo- and spermatogenesis, and 

 spawning. 



II. EMBRYONIC PERIOD. This extends from fertilization to 

 the opening of the mouth. This phase of development is very 

 rapid: at normal seasonal temperatures of the water it may 

 occupy only thirty-six hours. The entire period is con- 

 veniently subdivided into two. 



A. Before Hatching. This includes fertilization, cleavage, 

 gastrulation, and the first mapping out of the embryo. The 

 conclusion of this period is marked by escape from the egg 

 membranes eight hours or more. 



B. After Hatching. Here the chief systems and organs of 

 the embryo become definitely laid down. The embryo is free 

 swimming (pelagic) twenty-eight hours or more. The actual 

 duration of these and the other periods depends chiefly upon 

 temperature; it may be nearly doubled in the cooler waters of 

 an early season. { 



III. LARVAL PERIOD. From the opening of the mouth to 

 the cessation of pelagic life and the assumption of a burrow- 

 ing habit. Here development is slower and consists largely in 

 the elaboration of the parts marked out during the preceding 

 period about three months. At the conclusion of this period 



