18 PETER OKKELBERG 



along one side. The two germ-gland anlagen have not yet 

 fused at any place along the midline, although they approach 

 each other very closely at their cranial ends. 



h. Larva 373| hours old. The germ-gland anlagen extend 

 farther forward than in the preceding stages. Since the large, 

 inert, yolk-bearing cells are very poorly adapted for independent 

 migration, it is probable that their movement craniad as well as 

 mediad is due, at least in part, to the mechanical shifting of the 

 parts surrounding them. A coelom had formed in the cranial 

 portion of the mesoderm and is also forming in the caudal 

 region in front of the anal opening. In the middle portion, 

 however, no body cavity is yet formed. The gut is loaded with 

 yolk and is surrounded by mesoderm. In this mesoderm, on 

 the doral side of the intestine, the yolk-laden germ cells occur, 

 sometimes singly and sometimes in groups. Cranially, the two 

 lateral germ-gland anlagen are well defined and practically come 

 together. The cells are greatly flattened dorsoventrally by the 

 pressure of the surrounding tissues. Caudally the germ cells are 

 scattered along the whole lateral plate. In a cross-section from 

 the caudal region slightly craniad of the anal opening, a large 

 germ cell (fig. 21) was found lying in what may be considered 

 the ventral mesentery. It is highly probable that a germ cell so 

 situated will never become functional. 



i. Larva 429| hours old. The posterior cardinal veins have 

 appeared at this stage and the germ cells lie ventrad of these 

 cranially (fig. 22). As the coelomic cavity is being formed by a 

 sphtting of the mesoderm, the germ cells become included in the 

 somatic portion (fig. 23). 



j. Larva 478| hours old (4iV mm. long). At this stage the 

 germ-gland anlagen have fused cranially (fig. 24) ; caudally they 

 still remain apart. The cardinal veins have increased in size 

 and now lie dorsomediad of the pronephric ducts. Cranially 

 the mesenchymal tissue has increased greatly in amount in the 

 region in which the germ cells are found. It fills a considerable 

 space between the germ cells, the dorsal aorta, and the cardinal 

 veins. The germ cells are still flattened against the gut ven- 

 trally. They still retain their embryonic structure and are not 

 dividing. 



