6 OUTLINES OF CHORDATE DEVELOPMENT 



rather spherical head is about 1 micron in diameter about the 

 size of a deutoplasm granule. 



The gonads are metameric organs distributed through the 

 middle and posterior pharyngeal region. There are about 

 twenty-six pairs (twenty-three to twenty-eight), approximately 

 in segments ten to thirty-six. They first begin to develop in the 

 embryo before the mouth is opened, and apparently first come to 

 maturity when the organisms are 2 to 2.5 cm. in length. The 

 discharge of the germinal products involves the nearly complete 

 loss of the gonads as such, so that after each annual spawning 

 period they redevelop from small rudiments. Details of the 



AD 



PV 



FIG. 2. Diagram of a median sagittal section through the fertilized egg of 

 Amphioxus at the stage of the fusion of the egg and sperm pronuclei. After 

 Cerfontaine. The arrow marks the direction of the chief or polar axis. AD, 

 Antero-dorsal region; n, egg and sperm pronuclei; p, region relatively free from 

 deutoplasm; PV, postero-ventral region; s, remains of tail of spermatozoSn; 

 y, yolk or deutoplasmic bodies; //, second polar body. 



development of the gonads will be described later. The par- 

 tially grown ovary, which the testis resembles in essentials, is a 

 large spheroid mass, projecting from the postero-ventral margin 

 of the segment into the atrium, covered of course by atrial 

 epithelium. Fig. 3 represents in diagrammatic form a section 

 through the half grown ovary in the so-called medusoid or 

 mushroom stage. An abundant vascular supply is derived 



