504 STANLEY C. BALL 



His figures of the morulae of Bothromesosotomum personatum 

 and Mesostomum lingua are similar, with slight characteristic 

 variations. Although the blastomeres in the segmentation 

 of the egg of Plagiostomum girardi separate remarkably from 

 one another, Bresslau finds them later drawing together to form 

 a compact mass. 



The morula of P. gemellipara lacks the marked bilateral 

 symmetry due to a sagittal cleft which, according to Bresslau, 

 characterizes a corresponding stage in the development of 

 Mesostomum ehrenbergi. In P. gemellipara it has been 

 found impossible to distinguish the dorsal from the ventral sur- 

 face of the morula. The point of view in figure 15 is from almost 

 directly anterior; any attempt to distinguish here between 

 dorsal and ventral is clearly of no avail; the cells are all alike. 

 On the lower left, however, the posterior primary entoderm 

 cells are differentiated by their larger size. 



d. Homologues of the germ layers. While it is true that, as 

 Bresslau says of Mesostomum and other Turbellaria, no defi- 

 nite germ layers are formed in these worms, at least rather 

 distinct germ regions are discernible. As the development 

 of the morula progresses there are produced as derivatives 

 of the one macromere about sixteen blastomeres which receive 

 a portion of the mitochondrial mass. They form a cup-like 

 layer, one cell thick, over the surface of the morula which 

 lies toward the center of the yolk-mass, and serve, as will 

 soon become clear, to indicate the polarity of the embryo; this 

 cap of cells is posterior to the rest of the morula, and is 

 to be looked upon as constituting the primary entoderm. Two 

 other regions can be distinguished in the morula at this stage, 

 (fig. 16), first, the mes-ectoderm consisting of smaller cells 

 which are conceived to be direct descendants of the micromeres, 

 and second, a layer of larger cells with less dense cytoplasm 

 lying between the primary entoderm and the mes-ectoderm. 

 These are derivatives of the second macromere, seen in text 

 figures 11 and 13, which lacked the mitochondrial mass. Since 

 they ultimately give rise to the intestine it is appropriate to 

 call this layer of cells the secondary entoderm. 



