DEVELOPMENT OF PARAVORTEX GEMELLIPARA 491 



sheet over the surface of the yolk syncytium. Later the definitive 

 ectoderm, arising also from derivatives of the blastomeres, is 

 laid down beneath the first. 



To summarize: Bresslau ('04) observed the development of 

 a distmct shell membrane about the yolk mass in the capsule 

 of Mesostomum ehrenbergi. It was derived by a differentiation 

 of the outermost vitellarial cells. In Mesostomum lingua and 

 Plagiostomum girardi he found occasional nuclei lying in small 

 masses of cytoplasm at the periphery of the yolk spheres, while 

 in the case of Bothromesostomum all of the yolk nuclei degener- 

 ated; none showed any tendency to form a membrane at the 

 surface of the vitelline mass. On the other hand Mattieson 

 ('04) determined that a 'primary ectoderm' was established 

 about the yolk-syncytim of Planaria torva, P. polychroa and 

 Dendrocoelum lacteum by the union of migatory blastomeres. 



According to Hallez's interpretation, certain of the yolk-cell 

 nuclei in Paravortex cardii not only form a peripheral syncytium 

 outside the embryos and the nutritive portion of the yolk, but 

 later enter into the formation of the permanent ectoderm as 

 well as the primary intestine. 



The behavior of the yolk cells enclosed in the capsule of P. 

 gemellipara resembles that in the case of Bothromesostomum 

 personatum in that all their membranes immediately disin- 

 tegrate. The process is so rapid that in two hundred and seven- 

 teen worms sectioned only one capsule (fig. 11) was encounted 

 where a considerable part of these cell membranes remained 

 mtact. As in Bothromesostomum also, the yolk-cell nuclei 

 at once begin to degenerate. Ordinarily they have disappeared 

 altogether by the time cleavage is well under way, but occasionally 

 degenerate masses of chromatin embedded in small islands of 

 nucleoplasm remain visible until the primary entoderm cells 

 have been differentiated. No membrane or temporary ecto- 

 derm is formed by the peripheral yolk cells. 



The yolk in perserved material, whether enclosed within a 

 cell membrane or lying free in the capsule, consists of multitudes 

 of small globules and flakes lying in a clear plasma. On the 

 whole, the appearance of the yolk in a newly-formed capsule 



