THE TURBELLARIA 



yolk cells, and the embryo now becomes greatly swollen ; this 

 pharynx is, however, temporary, and disappears along with the 

 mouth. This, perhaps, is the remnant of some larval stage, for 

 the embryo is ciliated and moves within the cocoon. Later, a 



FIG. XIV. The Development of Planaria lactca. (After Hallez's figures and 

 description.) 



1. The egg cell (o) surrounded by some amoeboid vitelline cells (v). 



2. Segmentation ; stage with four blastomeres ; the vitelline cells are losing their 

 independence, and are forming a syncytium, represented by the small circles (y), in which 

 the blastomeres are embedded ; the blastomeres (bl) become completely separated from one 

 another ; n, nuclei of yolk syncytium. 



3. Later stage in segmentation ; one of the blastomeres is represented in division. 



4. A much later stage, showing the differentiation of the blastomeres into (ep) epiblast, 

 which comes to surround the yolk syncytium ; w, wandering cells moving in the syncytium ; 

 hy, a group of four hypoblast cells, destined to further subdivide and give rise to the wall of 

 the enteron ; and (ph) the provisional pharynx which marks the anterior end of the embryo. 

 A few vitelline cells still remain around the embryo. 



5. The cell differentiation has gone further and the epiblast (ep) completely surrounds tha 

 yolk syncytium (which in this figure is left plain). The four cells of the hypoblast (/< y) have 

 further subdivided, and now surround the enteron (ent). The pharyngeal cells have also 

 undergone similar changes, and enclose a cavity communicating with the exterior ; some 

 vitelline cells (v) have been taken through the mouth into the enteron, and are represented 

 by dotted circles (V) ; some of the wandering cells (/) have placed themselves round the 

 pharynx to form its outer layer, and no doubt give rise to its muscles. 



6. The embryo at a later stage changes its shape from a sphere to a flattened ovoid ; the- 

 definitive pharynx is being formed from the mass of cells, and the mouth is about to form by 

 the rupture of the lower boundary of the pharyngeal cavity. From the migratory cells (w) will 

 be formed the parenchymal tissue and generative organs. 



new mouth and a new pharynx, said to be hypoblastic in origin, 

 replaces the temporary one. The enteron is at first rhabdocoelous, 

 the various caeca resulting from ingrowths of connective tissue 

 septa. Some species of Planaria multiply by fission, preceded in 

 some cases (P. albissima) by the formation of a new head. 



