18 



Bojanus* and V. Baerf trace the development of the 

 cercariform individuals from the ( king's-yellow worm/ 

 which in its form and simple structure corresponds with 

 the vermiform offspring of the first ciliated embryo. 



Nitzsch J traces the cereariform progeny of that worm to 

 their pupa state. 



Steenstrup , confirming the origin of the Cercarise from 

 the multiparous ' king's-yellow worm/ completes the ob- 

 servation of their metamorphosis through the pupa-state 

 into the trematode Entozoon. 



The conditions of structure to be considered in the at- 

 tempt to explain the successive parthenogeneses are the 

 following. 



In the Distoma tereticolle \\ the first germ-cell, which is 

 the result of impregnation, multiplies itself in the usual 

 geometrical ratio, progressively absorbing and assimilating 

 the germ-yelk, until a germ-mass of countless minute cells 

 is formed ; but the subdivision and diffusion of the sper- 

 matic force is carried further by the reduction of the 

 progeny of the primary germ-cell to the size and apparent 

 state of nuclei or granules ; in which state, with a clammy 

 fluid vehicle, inclosed in ( a vermiform sac, usually composed 

 of a ciliated integument and sometimes provided with one 

 or more eye-specks, the germ-mass quits the ovum. We 

 perceive, therefore, that it has undergone no other change 

 than that which has contributed to the formation of such 

 peripheral parts. There are no proper internal organs. In 

 the Monostoma mutabile it would seem that the true nu- 

 clear matter of the germ-cells had, after their ultimate sub- 

 division, aggregated itself together in the centre of the cili- 

 ated embryo or modified germ-mass. In that aggregated 



* Isis von Oken, 1818, p. 729- 



t Beitragezur Kenntniss der Niederen Thiere, Nova Acta Leop. Car. 

 torn. xiii. pt. ii. p. 629. 



J Beitrag zur Infusorienkunde, Halle, 1817. 

 On tha Alternation of Generations, p. 52. 

 II Kolliker, Miiller's Archiv fiir Physiologic, 1843, p. 99- 



