N o. I .] POL YCHOER US CA UDA TUS. 7 9 



Growth of the Ovum. 



Several methods of killing and hardening were used, but by 

 far the best results were obtained after using Hermann's fluid. 

 No other reagent seemed to preserve the nuclear structure so 

 satisfactorily. Pretty good results were also obtained with 

 Flemming's fluid, and also with weak formaline, but corro- 

 sive sublimate solutions, either with or without the addition 

 of acetic acid, gave but poor results. As related in a former 

 paper (10) on the early development of this form, no satisfac- 

 tory method of killing the ova after they were laid in their 

 capsules was found. This is probably due to the impenetra- 

 bility of the capsule. 



In his monograph on the Rhabdocoela, von Graff (8) states 

 that in the Acoela no vitellarium is present ; and in this Dr. 

 Sophie Pereyaslawzewa (3) agrees. In a later work (9) he 

 describes with some detail the rapid growth of the ovum from 

 the small germ cell as being due to one germ cell absorbing 

 the substance of its immediate neighbors, which are thus con- 

 tinually reduced in size, while the only germ cells destined 

 to reach maturity grow at their expense. Thus, he says, 

 a struggle for existence and survival of the fittest occurs 

 among germ cells just as among individuals. That this 

 is the method of growth of the ova in many Acoela I have 

 satisfied myself by sectioning specimens of different spe- 

 cies ; but in P. caudatus it is quite different, for, when very 

 small, the ova pass from the ovary into an enlarged and dif- 

 ferentiated portion of the oviduct, which is charged with food 

 material, which the ova there absorb. Professor Mark (i) 

 speaks of this as a " differentiated portion of the ovary, where 

 the cells destined to reach maturity undergo . . . rapid increase 

 in size." I think this should be described rather as a differen- 

 tiated portion of the oviduct ; for, as PI. IX, Fig. i (i>t.), shows, 

 it lies between the ovary and the female genital pore. This 

 Fig. I is a diagram showing the condition of the female repro- 

 ductive organs at different seasons. The left half shows the 

 ovary (pv.) and enlarged oviduct filled with vitelline or food mate- 

 rial (yt), in the spring or early summer, before the near approach 



