526 STANLEY C. BALL 



ectoderm contained two small globules, of the same consistency 

 and color as the food material in the entoderm. It is possible 

 that through the agency of mesenchymatous cells the ectodermic 

 nutriment is passed on to the tissues of the embryo. On the 

 other hand this cell may have belonged to the entoderm, its 

 contents representing the last stage in the digestion of the 

 entodermic yolk. 



During the absorption of the reserve food, the ectoderm, or 

 external epithelium of the young worm, has undergone vacuoli- 

 zation; the cytoplasm of each cell now forms trabeculae between 

 irregular clear regions (fig. 37). This condition is at first more 

 noticeable at the anterior end of the body where less yolk is 

 deposited, but with the absorption of the latter posteriorly the 

 whole ectoderm takes on this appearance. This vacuolization 

 causes a marked increase in the depth of the cells. 



h. Formation of the intestine. According to Bresslau ('04) the 

 intestinal epithelium is formed in Mesostomum by previously 

 undifferentiated embryonic cells which unite first in the region 

 just posterior to the esophagus. Gradually this sheet of cells, 

 by the addition of others, extends posteriorly about a cavity 

 which arises as a splitting of the tissues in the region of the fast 

 shrinking yolk cells. At the same time the intestinal cells be- 

 come vacuolated. 



Hallez ('09) asserted that the cells which enter into the for- 

 mation of the intestine of Paravortex cardii are of the same nature . 

 as those which give rise to the mesenchyme. He considers 

 their immediate predecessors to be large cells lying at the pos- 

 terior end of the body. These proliferate at once cells which 

 are added to the intestinal 'syncytium,' and others which con- 

 stitute the mesenchyme between the intestine and body wall. 

 He observed that a lumen appears first in the region just behind 

 the oesophagus, and gradually, with the absorption of the vitel- 

 line spheres, extends posteriorly. 



From certain observations of the conditions under which the 

 intestine arises in Paravortex gemellipara* there is some doubt 

 that it originates in the manner described by Hallez for P. cardii. 

 It seems rather that the large posterior cells, as noted in the 



