THE TURBELLARIA 



39 



stomodaeum, which will give rise to the pharyngeal sac and 

 pharynx. 



5 



FIG. XIX. 



1. Eggs of Yungia laid in a single sheet, embedded in jelly ; eacli egg is provided with an 

 operculum (a), which is represented in various positions, and entirely removed. 



2. An egg of Thysunozoon. s, the egg membrane ; a, coarsely granular vitellus ; Z>, fine 

 granular vitellus, containing the nucleus. 



3, 4, 5. Three stages in segmentation of Discocodis. The result of the first two cleavages 

 is to produce four large cells, each of which then divides into a micromere (k) and a macromere 

 (t/). These first formed cells divide and the macromeres also divide, forming mesoblast On). 

 5 represents a later stage from the opposite (ventral) pole, and shows the four primary eudo- 

 derm (hypoblast) cells (e) derived from the four large yolk cells. 



6, 7. Diagrammatic transverse sections of later stages. 6 represents the same stage as 5. 

 The epiblast (fc) is extending over the mesoblast, in. There is a small blastocoel, represented 

 black ; e, the primary hypoblast cells. 7 is a much later stage, after the epiblast has grown 

 right round the embryo, so as to enclose the cells, leaving however a small blastopore, 6. In 

 addition to the primary, ventral, hypoblast cells, others (/) have been formed from the yolk 

 cells dorsally. By the subdivision of these hypoblast cells (e and /), the great yolk cells 

 will become enclosed ; the nucleus is no longer distinguishable, and the yolk spherules run 

 together to form a great homogeneous mass occupying the cavity of the gut, and serving as 

 food. The mesoblast in this stage has already extended downward for some distance. 



8, 9 represent two stages in the development of the larva of Thysanozoon. a is the brain 

 with eye-spots ; 1 to 8 are the eight characteristic ciliated lobes which are so well marked in 

 Miiller's larva ; 1 is the median, supraoral, anterior lobe ; 2, 3, the paroral lobes ; 4, 5, great 

 lateral lobes ; 6, 7, pair of posterior lobes ; 8, median posterior lobe. 



8 is a side view of a young larva. 9 is ventral view of an older layer. TO, mouth, the light 

 area around which is the pharynx. (All after Lang.) 



The embryo is now somewhat ovate ; as the yolk is absorbed 

 and the animal grows it becomes flattened, assumes the form of a 

 young Polyclad, and leaves the shell. 



