490 , STANLEY C. BALL 



The development of M. lingua and M. productum is so similar 

 that Bresslau figures only the former. While a limited number 

 of yolk-cell nuclei come to lie next to the shell the others rapidly 

 disintegrate. No distinct shell membrane is indicated 



The newly formed capsules of the Alloiocoele, Plagiostomam 

 girardi, are stated by Bresslau to contain from ten to twelve 

 eggs distributed amongst several hundred yolk cells. The 

 latter then arrange themselves in groups about the several eggs. 

 In the behavior of the yolk-cell contents a notable difference 

 is to be observed in comparing it with the process of yolk trans- 

 formation in Mesosotomum. The vitelline substance consists 

 of a large number of small refractive spherules which collect 

 at the periphery of the cells. Gradually the cell membranes 

 disintegrate, the process setting in first next to the egg. Since 

 the nuclei, like those in M. ehrenbergi, remain distinct for some 

 time a yolk syncytium results. Vacuolization does not occur 

 until after the embryonic ectoderm has begun to grow out over 

 the yolk. As in three of the four species of Mesostomum a 

 small number of yolk-nuclei become flattened on the surface 

 of the vitelline mass, while the remainder, although less con- 

 spicuous than at first, remain visible, scattered through its 

 interior until the ectoderm has enveloped the yolk. Here again 

 no shell membrane is developed form the yolk cells. 



For the Dendrocoeles, Planaria torva, P. polychroa and Den- 

 drocoelum lacteum, Mattieson ('04) found that about one hun- 

 dred and fifty of the yolk cells immediately surrounding the 

 egg become separated from those farther removed and pass 

 over into a syncytium. This change takes place before the 

 enclosed embryo has reached the two-celled stage. Mattieson 

 concluded that, even previously to the formation of the syncy- 

 tium, an active interchange of material between the egg and 

 yolk cells takes place. The nuclei of the yolk cells which enter 

 this syncytium slowly degenerate, but are still met with at the 

 stage when the pharynx is differentiated. 



Two ectoderms are developed in these Dendrocoeles. First 

 a transitory epithelium is formed by blastomeres which migrate 

 through the yolk and arrange themselves in an extremely thin 



