The life history of Diplodiscus temporatiis Stafford. 641 



Haswell (1903) lias pointed out that in other forms {Ecliino- 

 stomum) the same appearance of the nuclei is often noted in early 

 stages of segmentation. He states, however, that they are not 

 restricted to the central part of the embryo, and suggests that the 

 characteristic appearance of these nuclei is to be accounted for, 

 not on the supposition that they differ from those of the surrounding 

 cells, but that they represent a certain stage in the usual cycle of 

 cell development. 



My own observations on Diplodiscus are in perfect accord with 

 those of Haswell, In any section containing young embryos it is 

 possible to find many nuclei similar to that shown by Schwaeze 

 in his fig. 1. In a preceding section of this paper it has been 

 pointed out that the densely staining stage of a nucleus is a cha- 

 racteristic step in the cycle of events leading up to its division by 

 mitosis. Such an appearance does not, therefore, afford any certain 

 basis lor the assertion that the nuclei noted by Schwarze are 

 different from their neighbors. 



Looss (1892) states that the first of the permanent organs to 

 appear in the young cercariae of Amphistomum subdavatum is the 

 water vascular system. He figures (fig. 11 tab. 20) the two main 

 trunks of this system as extending nearly the whole lenghth of the 

 body, which as yet consists only of an undifferentiated mass of 

 "meristem cells". 



RossBACH (1906), in the passage previously quoted, recognizes 

 the gut as the first of the permanent organs to be laid down. He 

 states that the pharynx makes its appearence at about the same 

 time. 



Tennent (1905) notes first the appearence of a number of large 

 cells which form the primordium of the pharynx. 



In the embryos of Diplodiscus, developing from parthenogenetic 

 eggs, the primordium of the gut often remains recognizable from 

 the time when the single macromere is left after the cutting oft" of 

 the two micromeres. In embryos where the limits of the primary 

 germ layers are indistinguishable (Fig. 55), the first organ diffe- 

 rentiation to be recognised is the assembling of the endoderm cells 

 to form the primordium of the gut. A definite arrangement of some 

 of the ectoderm cells at the anterior end of the body to form the 

 primordium from which the anterior sucking disc, and the ecto- 

 dermal portion of the digestive tract, is derived, takes place at 

 practically the same time. 



