A Coutribut. to the Embryol., Life-histoiy, and Classificat. of the Dicyemids. 37 



which produces this corpuscle does not , apparently, develop directly to 

 a Dicyemid , but produces g-erm-cells which so develop. I shall bave 

 occasion to return to this objection further on. 



We may now trace the origin and fate of this corpuscle , and the 

 history of the celi from which it arises. The earliest stage in its forma- 

 tion with which I bave met is given in figure 104, PI. 5. This figure 

 represents apparently a nearly fini shed karyokinetic di vision, in 

 which ali, or nearly ali , of the ])rotoplasni remains with one of the 

 daughter nuclei. The partially liberated portion [n) is transparent, 

 slightly granular , and shows a large nucleolus-like body in its exter- 

 nal surface. The evideuces of a karyokinetic division were more 

 plainly indicated in the originai drawing than the lithographer has 

 represented. The piane of division was marked by what appeared to 

 be short remnants of spindle-fibres, directed at right angles to it (incor- 

 rectly represented by mere dots in the figure^ . The fully liberated cor- 

 puscle, hemispherical , and stili in contact by its flattened face with the 

 producing celi, is seen in fig. 105. It is, in this instance, slightly larger 

 than the centrai nucleus, and shows no distinctly defined membrane. 

 It has ali the appearance of being a nuclear body, and its subsequent 

 history shows that it may be so regarded. It is not improbable that it 

 contains a very small amount of celi protoplasm , and that from this is 

 form ed the thick membrane that arises soon after it becomes free. Be 

 this as it may, it will later be impossible to distinguish this body from 

 others of an undoubted nuclear character, and, as it remains for a con- 

 siderable time near the proliferating celi, it may conveniently be called 

 a paranucleus. 



In fig. 106 (from a young Dicyema typus) the paranucleus [n) lies 

 detached by the side of two cells [g] which bave arisen by division of 

 the celi m in fig. 105. Fig. 107, from a D. moschatum^ .5 mm long, 

 represents a similar stage , in which the paranucleus already shows a 

 double-contoured membrane. The multiplication of the two cells [g in 

 figs. 106 and 107) by division soon leads to the formation of a cell- 

 aggregate, such as is seen in figs. 108 and 109 </. 



At this time one of the cells, generally occupying a centrai posi- 

 tion, as seen in fig. 109, and in Van Beneden's figures 15 and 21, 

 PI. 1, is larger than the others, and incloses a nucleus [n") that takes a 

 sharper outline in acetic acid than is seen in the nuclei of the peripheral 

 cells. This celi becomes the generatrix of successive generations of 

 cells . ali of which , if we except those of the last generation , are des- 

 tined to produce infusoriform embryos in the manner described by Van 



