1904.] NATURAL SCIENCES OF PHILADELPHIA. 703 



rivetl from the intermediate girdle cell of the D quadrant. IMead (1897), 

 Treadwell (1901) and Torrey (1903) have shown that in the annelids 

 Am'phitrite, Podarke and Thalassema respectively cells from this region 

 wander out through the dorsal gap of the prototroch and contribute to 

 the dorsal ectoderm of the trochophore. The thinness of the dorsal 

 ectoderm of the Dinophilus embryo after the closure of the blastopore 

 bears witness to the great mitotic activity in this region (compare figs. 

 54 and 56 and fig. 58). While this forward movement is taking place the 

 ectoderm is increasing in lateral extent, as comparison of the embryos 

 illustrated in text fig. IV readily shows. At the same time on the ven- 

 tral side cells of the ventral plate have covered over the mesomeres, 

 so that now the entoblast is completely enclosed with the exception 

 of a small area at the vegetal pole, the blastopore. This soon closes, 

 the ectodermal cells fusing so smoothly as to leave no visible trace of 

 their union. Meanwhile the entodermal cells have undergone a 

 change in that their lower or vegetal pole ends have become progres- 

 sively smaller as the blastopore closes, while their upper or animal pole 

 ■ends have become correspondingly larger. During this change in 

 shape of the entomeres their nuclei recede from the surface and move 

 inward (figs. 53-56). 



The stomodseum appears as a shallow depression of the ectoderm 

 at the point where the blastopore closed (fig. 58). The ventral 

 plate now begins to grow forward very rapidly, pushing the stomo- 

 dseum before it until the latter reaches a subterminal position, the 

 position of the definitive mouth (fig. 59, st.). The stomodaeal invagina- 

 tion now deepens from a shallow depression to a finger-like inpushing, 

 directed somewhat backward. In fig. 59 the posterior wall of the 

 stomodseum is seen to have thickened ; this mass of cells (pro.) is the 

 rudiment of the proboscis. In fig. 60 this organ (pro.) has assumed 

 essentially its definitive structure and relations. From the stomodseal 

 invagination is formed all of the alimentary canal anterior to the 

 stomach, including the Vormagen or proventriculus. 



While these changes are taking place the entodermal cells have been 

 slowly dividing and have arranged themselves about a centrally situ- 

 ated cleft, the rudiment of the lumen of the future stomach and intes- 

 tine (fig. 59, sto.l.). As the development progresses the entodermal 

 cells assume the arrangement and appearance of an epithelium, while 

 at the same time the lumen of the future stomach increases in extent. 

 In fig. 60 the entoderm cells are now seen to form a cuboid al epithelium, 

 while the lumen of the stomach is a long and narrow cleft, extending 

 obliquely downward and forward. From the posterior end of the 



