(340 l^- R- ^'•^R'^- 



"Hüllmembran" represent the first indication of cell differentation. 

 seems to me to be explaned on the ground that, as is shown in his 

 fig. 1, the differentation of the germ laj^ers has already taken place 

 for the most part in an earlier stage in the segmentation. His 

 figures show also that all cell outlines have been lost, so we maj' 

 conclude that his material was unfavorable for the demonstration 

 of this point in the development. 



His figures of the early stages in the development of the 

 parthenogenetic eggs (germ balls) within the redia show, however, 

 that here at least, the first tw^o segmentation divisions result in the 

 cutting off of two smaller cells from a larger one. This process is 

 in itself the separation of the primitive ectoderm from the primitive 

 endoderm, and it seems most probable that the same process took 

 place in the development of the redia. 



The entire absence of cell outlines, or even a separation of the 

 cell layers, in most of the embryos, has made it impossible to trace 

 the cellular history of any of the organs of the larvae with the 

 exception of the gut and the investing membrane. In the early 

 development the only material which has suggested a possibly re- 

 cognizable formation of the mesoderm is shown in Fig. 54. Here the 

 cells forming the endoderm are surrounded by two concentric cell 

 masses which are sharply separated from one another. The outer- 

 most of these surrounding layers is plainly the ectoderm (compare 

 Fig. 53), the inner layer may repi*esent the beginning of the meso- 

 derm; although even if this is admitted, there is not the slightest 

 evidence to show from which of the primary germ layers it had 

 its origin. 



The development of the c e r c a r i a. 



There are some striking discrepancies in the observations of 

 previous investigators in regard to the order, and, in many instances 

 to the manner, in which the systems of organs become differentiated 

 in the development of the cercaria. 



According to Schwarze (1886) the first differentation noticeable 

 in the development of the cercaria takes place when darkly staining 

 nuclei appear in the interior of the segementing germ ball. These 

 nuclei are most numerous in the central part of the morula, and 

 are interpreted as constituting the primordium of the reproductive 

 sj'stem. These cells begin to be differentiated before the morula 

 has been overgrown by the investing cells. 



